Muse from Romancing SaGa 3. Surprisingly enough, she is one of the better characters to recruit after she is cured of her sickness.
Subverted in Super Robot Wars Original Generation: Ryusei's mother is an adult Ill Girl, but it's due to a military project she used to work on as a psychic destroying her body. Her son has ended up being "recruited" by the same project in exchange for paying for her hospital bills.
Silent Hill 2 has Mary, a rare adult example. Naturally, she has the Incurable Cough of Death, too. The real rarity is the fact that she dies before the start of the game and is only seen and heard in flashbacks or maybe not.
Breath Of Fire 3 subverts this during the Contest of Champions. Emitai, one of the competitors, visits the heroes before their bout (which is weighed against him due to its rules) and introduces them to his daughter; he claims to be in the contest to earn her an operation that will cure her disease, and asks them to throw the fight. As one of the heroes is being held hostage, they refuse. After the match, however, if you visit Emitai's dressing room, you learn that it was a giant fraud on his part to get his opponents to forfeit. Just to make this clear, after the midgame time skip, you can find Emitai again (and recruit him as a master); his daughter has grown up and is rather embarrassed with him.
In Advance Wars: Days of Ruin, Isabella/Catleia fills this role early on due to being infected with the mysterious floral virus. Will/Ed is the one to cares for her most, but because the virus only affects younger people, he'd be in danger if he was around her too much. Due to her strange memories, the crew manages to find a cure for it. Unfortunately, the virus later evolves to be able to affect matures as well.
Heather the Ghoul from the Vampire: The Masquerade Bloodlines game practically embodies this trope. First, you have to save her life when she lies dying in a hospital. Then, when she finds you again, she acts so cute and affectionate, you just have to have a heart of stone to send her away. Then, she starts making you small presents and other pleasant things, like letting you feed on her blood, free of charge, etc. Most of her behavior is determined by her condition (by turning her into your ghoul, you bind her to yourself and your blood becomes a powerful, addictive drug for her) but that doesn't diminish her cuteness even one bit. Trust me. That she is a Meganekko and a potential Cosplay Otaku Girl doesn't exactly help, either... if you don't send her away, at some point she is gonna be kidnapped and murdered as an act of revenge upon you.
Subverted by Raquel in Wild ARMs 4. While she's slowly dying of an unknown disease in her body that's greatly implied to be radiation poisoning, it doesn't stop her from becoming a badass swordswoman and one of the most powerful characters in the game.
Tyler Chase's little sister Amy is infected with the Deftera GUILT two years prior to the first Trauma Center, and eventually goes into critical condition in Chapter 3. Derek operates on her and cures her, thus removing her status as Ill Girl.
Colette from Tales of Symphonia, who ends up suffering from a couple various ailments, the first being Angel Toxicosis Actually is pain from her soul being consumed by the Cruxis Crystal, as part of the World Regeneration Journey, and the second Chronic Angeleus Crystallius Inofficium actually does disfigure her (In the one cutscene where it's visible) and you spend a while (half of disc 2) getting the cure for her.
Cheria starts Tales of Graces like this, but she gets better when Asbel returns.
Tomoki in Canvas 2. However, the surgery she needs is actually quite easy and not that expensive. Not even that risky. She's actually afraid it will work and she'll be lonely.
Mary/Maki Sonomura in Persona is introduced as an Ill Girl, but when demons arrive to the scene, she suddenly got back up. Turns out this healthy Mary is the 'Ideal Mary', imagined by the real, ill Mary, who dreamed of her own world where one of the aspects there is that she's healthy, and by some complicated means, it's coming to take over reality.
Yasumi Aizawa in Aoi Shiro is one of these. Until it turns out she just needed a little blood.
Jessica from Mana Khemia: Alchemists of Al-Revis. One alchemist, who turns out to be The Hero's "father", sort of, cured her ... at the cost of her life energy. In short, she got better, but her life was cut in half at least.
Fana from Avalon Code. Interesting in that you can actually heal her by removing the illness code attached to her, but this requires a series of related plot events, as you can't just pluck out the code and slap it on something else (codes with this property are marked with spiked borders).
In the Tokimeki Memorial series, there's MioKisaragi of Tokimemo 1, who suffers from anemia and thus and can't handle violent physical activities and emotions, and Hotaru Izumi of Tokimemo 3, who had to stay for a long time at the hospital, and still suffers from some aftereffects, due to a car accident a few years prior the game's proper, which also cost the life of her dear boyfriend.
In Mitsumete Knight, the Spiritual Successor of Tokimeki Memorial, the resident Ill Girl is SarahPixis ; she suffers from a heart disease that prevents her to go outside, and the Asian (the player avatar) gets to meet her as his private teacher. Other Ill Girls are Sophia'sestranged mother and little brother, Dolphan's Queen, as well as Teddie Adelaide, Sarah's nurse and friend, who suffers from the same heart disease as her and thus why the two are close.
Yonah from NieR, who is infected with the Black Scrawl Virus, prompting her father/brother (depending on which version you buy) to go out and fight his way to finding a cure. Later on, it is revealed that Kaine is also infected.
Patchouli Knowledge from Touhou is really sick. She suffers from asthma, anemia, and Vitamin A deficiency because of her refusal to leave the library she lives in for centuriesnote and also because of all the poisonous chemicals, such as mercury and arsenic, that she uses for her magic. This is used to justify her inconsistent power level; when she's the midboss of the Extra Level in Embodiment of Scarlet Devil, she explains beforehand that her asthma has cleared up.
In the JRPG Lost Odyssey the player can acquire an ingame codex entry of the maincharacters past experiences. One in particular centers around an Ill Girl whom liked to hear his stories of far off places when ever he stopped by. The codex ends with the last encounter with this Ill Girl has her comatosed and on the brink of dying when he arrives to tell her one last final story.
Occurs a couple of times in the Growlanser series:
Growlanser II: The Sense of Justice includes Charlone's younger brother in this role.
Harvest Moon: Magical Melody features Dia, a girl you can romance who is incredibly ill from some vague disease. As such she spends every waking hour in the hospital and you have to come visit her. You could always just woo her nurse instead.
Kaoru Watabe from The King of Fighters, who is in a wheelchair at the start of KOF'97. She has only started to take her first steps out of it by the time she meets her idols, the Psycho Soldiers. Around KOF 99 her health has improved dramatically, atleast enough to have her help the group out of the NESTS base.
In the Yume Nikki fangame .flow, through the game's symbolism, it is heavily implied that Sabitsuki, the player character, is dying from some sort of illness. This isn't just an Epileptic Tree, but almost canon. Maybe.
Maria Robotnik of the Sonic the Hedgehog series had NIDS. In fact, Gerald Robotnik's creation of Shadow the Hedgehog was his attempt to save Maria's life.
Miyuki Tanaka from True Love Junai Monogatari is, like Mio, anemic and prone to dizzy spells. She even collapses at some point and her friend Mikae has to take care of her.
Victoria F. Stein from Bravely Default is a dark twist on this trope; while she was subjected to an incurable disease, she was treated by Victor and his father Vincent. The treatment was imperfect, however, and while Victoria can move freely, not only did it stunt her growth, but she is subjected to increasingly frequent seizures, and the continued trauma has made her a different kind of sick.
Taimi of Guild Wars 2 is an Asuran child who suffers from a disease similar to multiple sclerosis, rendering it painful to walk for significant distances. She gets around this by riding in a combat-ready golem chassis.
Claudia, the main character in Legend of Fae has a leg brace due to contracting polio as a child. Fortunately, she can still kick butt using her affinities with the elemental spirits.
Dot Hack GU has Aina, Oban's sister who plays The World because it's the only way for her to communicate with her brother (He's in Japan, she's receiving treatment in Germany). She would've received a crucial treatment long ago... but unfortunately she became a Lost One (a victim of the PKer Tri-Edge. This greatly eats away at Oban's consciousness because he is Tri-Edge due to the AIDA infection. His plans throughout the series are so that he can free her mind from the game.
Em in Prelude is another adult example. She has an unspecified ailment which makes her tire easily and start coughing when she's been outside for too long and which prompted her father to discourage her from singing due to the vocal strain.
A Very Long Rope to the Top of the Sky: Mint, although, unlike more conventional examples, this doesn't stop her from adventuring with the rest, to the noticeable detriment of her health. Her condition is strongly implied to be genetic (or at least congenital), and the major way it manifests is that she will occasionally just collapse from too much excitement, strain, or some other strong emotion. She diesbecause of this.
Beatrix from Battleborn was born with a terminal disease that gave her a very weak body, a thing that is looked down upon by Jennerit society which greatly values strength. Because of this, she originally didn't have long to live. However thanks to her mother's lobbying to have Beatrix sustained for her genius, Beatrix became something akin to Deadpool, an immortal stuck with an otherwise sickly body. Excerpts from her diary from the lore in Battleplan 32 reveal her thoughts about her illness.
There once was a girl named Trix Whose body nobody could fix She went to the doctor Who stabbed her and stocked her With every pill in the mix.
The Big Good of the game, Queen Nanari of the Lilith Kingdom, has such a weak health that she hasn't been seen in public for years and her Number Two, Sir Nidhogg, has to handle almost everything. Even her members of her own family (like her younger brother, Prince Royce) can only visit with her on a limited basis. The Nanari that spirits the titular Nikki and her cat Momo to the world of Miraland is actually a product of Astral Projection, and disappears few later.
While not as frail as Queen Nanari, Nikki's Identical Stranger Bai Jinjin appears to have a weak constitution. When she and her boyfriend Zhong Lizi headed towards the frigid North Kingdom after the attempted kidnapping at the Designer's Tea Party, her health deteriorated to the point that they were forced to return to their homeland, the Cloud Empire.
Zhong Lizi: "The further north we went, the worse Bai Jinjin's condition would get. Maybe she just can't bear the freezing weather."
Growlanser II: The Sense of Justice has a female character whose younger brother is an ill boy. And yes, there is a mysterious "operation" that can cure him, and this provides that character's main motivation. In a slight variation on the usual plot, the character's family is very wealthy and can easily afford the operation, but the Ill Boy is afraid to go through with it because it is reputed to be extremely painful. (Maybe the Magitek of the setting doesn't include anesthetic?)
Then there's Prince Julius of Velthomer/Grannvale from Fire Emblem: Genealogy of the Holy War. In Fire Emblem: Thracia 776 he catches a high fever, and a conversation between his half-brother Saias and girlfriend Ishtar states that while he was somewhat delicate as a child, his health really declined after he became the vessel for the Dark God Loptous.
In Fire Emblem: The Binding Blade, Elffinaka Prince Mildian has really bad eyesight. He once was poisoned to near death, and this partial blindness is the only effect that still remains.
Subverted by Lucius, who is said to be pale and sickly. His companion Raven asks him once in a while if he's "under the weather", and he apparently has a borderline seizure in his C support with Renault, who also shows worry for his well-being. However, Lucius himself says it's not a physical illness, but one "from the soul". (It could be clinical depression or PTSD, as he says he tries to help others with their pain yet he is afflicted too.)
Played straight by Nils. At some point he does have a seizure and collapses, and his older sister Ninian asks the group not to move him from the spot he passed out on. The following mission has protecting him for 15 turns as its goal.
In the aforementioned Henry's supports with Olivia in Fire Emblem Awakening, he claims his bloodlust is a genetic nervous disorder. Whether it actually is or not, eh, it's something else. His profile, on the other hand, says that he actually has hypotension aka low blood pressure, which can potentially be quite serious.
In Mass Effect 2, assassin Thane Krios suffers from a condition called Kepral's syndrome, which many drell get since the comparatively moist environments non-drell inhabit causes their lungs, which are more accustomed to their homeworld's arid atmosphere, to gradually lose the ability to draw in oxygen. As a result of his condition, Thane joins Shepard's Suicide Mission readily: regardless of whether he joins, he may not have long to live.
A series-wide example would be the quarians, who cannot survive outside of their suits as they have a rather weak immune system, which was even further weakened by three centuries of living aboard sterile spaceships. they get better if you manage to get them to make peace with the geth during ''Priority: Rannoch'' and then choose the "Synthesis" or "Control" ending in Mass Effect 3 : since the geth do not believe in holding grudges, they welcome their quarian creators back to Rannoch with open arms and upload themselves to the quarians' suits to mimic a variety of diseases and illnesses. As a result, quarians are able to ditch the suits completely within a few years following the events of the trilogy.
In Mega Man Battle Network 3, a kid named Mamoru has an illness with a vague acronym (HBD) that essentially boils down to a heart defect. Lan helps the kid out and convinces him to go through another operation that is supposed to cure it because it just happens to be the disease that killed Lan's twin brother Hub, whose mind was digitized and placed inside Megaman.EXE. The hospital just has to be attacked on the day of the operation, and it's up to Lan to save everybody, including the kid.
Ion from Tales of the Abyss is known to have a weak constitution, and upon using a Daathic fonic arte, becomes weak to the point of collapsing. This is actually because the Ion seen in the game is a replica of the original Ion, who died a few years prior. Doing things such as using fonic artes or reading the Score cause Replica Ion's body to degenerate.
In Tales of Berseria, Velvet's younger brother Laphicet is rather sickly and often bedridden. ...Well, he was, until Artorius killed him. A sidequest reveals his illness was called the Twelve Year Sickness, inflicted progressively worse fevers on him every twelve days, and would have killed him anyway on his twelfth birthday. A late-game Wham Episode reveals he knew this, and volunteered to be Artorius' human sacrifice.
Emilio Juarez from Trauma Center: Under the Knife 2, who is one of the Sinners (read: tortured orphans used by the bioterrorist organization Delphi as lab rats) saved by Derek in the first game is seventeen, yet manages to be an excellent example of an Ill Boy all the same. If it weren't for all the "he"s and "boy"s used referring to him, though, it probably wouldn't be hard to mistake him for a more traditional example of the trope.
Subverted: Ukyo Tachibana from Samurai Shodown has tuberculosis (Incurable Cough of Death and Blood from the Mouth included), but he remains quite a badass fighter. Further subverted as the second game implies that he dies of it, but he reappears in the third. Amusingly, his Seppuku sees him cough and then collapse, as the TB finally finishes the job.
In stark contrast with his boisterous and muscular adult self, Dean from Harvest Moon: Light of Hope was a sickly child who spent much of his time indoors. He loved reading books about flowers so he begged his dad to take him hiking with him. Over time, the frequent hikes helped Dean become healthier.
Similar to the above, Hanbei from Pokémon Conquest suffers from tuberculosis (not like the game will state that). Unlike the above example, he keeps it to himself and does what Hideyoshi tells him to do, so at the end of his episode, he starts coughing. When asked what's wrong, Hanbei responds that he got too excited; Kanbei doesn't believe it one bit. In both cases, the disease eventually killed the real-life Hanbe Takenaka.
Akinari Kamiki, the Sun Arcana from Persona 3, is a prospective writer who was born ill, thus his days have been numbered ever since birth. Developing his S-Link is about having him learn to enjoy his last days of life. He will die before the game is over, leaving the children's book he wrote with the MC's help and encouragement as a Tragic Keepsake, and if his link is maxed his soul will cheer on you before you take on Nyx.
From the same game is party member Shinjiro Aragaki, who is slowly dying courtesy of suppressants that he's taking to suppress his Persona. It has degraded his immune system to the point that he gains a nasty Incurable Cough of Death and, according to Word of God, his body cannot regulate its body temperature properly anymore (this was given as an explanation as to why he always wears winter clothing during Spring and Summer.)
Daniel Curien, son of Dr. Roy Curien (an antagonist in the House of the Dead series) was overcome with an illness during his childhood days. Dr. Curien stepped beyond the boundaries, just to save his son's life.
Saki in Uncommon Time. It's subverted in that he's still capable of going adventuring with the heroes, but he's still very frail and sickly, and sometimes collapses and has to be taken care of. This is in fact very serious; he is terminally ill with a congenital disease and knows he's likely to die soon. He keeps this a secret from the others, but his Love Interest Meirin finds out anyway. In the Golden Ending, she promises to find a cure for him; in one of the bad endings, he chooses to commit suicide rather than let the disease catch up to him.
John Cassidy from Fallout 2 suffers from a heart condition, necessitating regular injections of cardio-boosters to keep from having a heart attack. If you give him a chem like Psycho, Buffout, or Jet, it will aggravate the condition to the point of giving him an instant, and fatal, heart attack.
In Dark Elf Historia, Ruse has always been a bit sickly as a result of the time he spent alone after his parents were killed, but if Fraylia manages to defeat Danny the Orc without failing any previous missions, Ruse will suddenly fall ill with "Neysterbey Syndrome", which requires Fraylia to immediately abandon paid questing in order to go find a cure.
In The Legend of Zelda: Twilight Princess, Prince Ralis of the Zora is first encountered gravely ill due both to being attacked by Zant's forces on his way to seeking help from Princess Zelda and to being dehydrated from being away from water too long. Link must escort Telma and Ilia as they take Ralis to Kakariko Village to be healed by the shaman there.
After regaining the memories of their true identity in Super Robot Wars X, Iori revealed to X-Cross members that they were originally somone who was physically weak, and prone to sickness. Which greatly surprised the X-Cross members.
Ensemble Stars! has Eichi, who has all the apparent theming for it, having pale blonde hair, a kindly disposition, and constant angel motifs. However, in a total deviation from the trope's usual implications, Eichi is actually a Bitch in Sheep's Clothing who presents an image as a kind Big Good type to the public while secretly sabotaging anyone standing in his way. To a lesser extent, there is also Chiaki who was sickly in his youth and spent a lot of time in hospital (crossing paths with Eichi several times), but who mostly overcame that illness as he grew up. However, he's still very accident prone, and so ends up in the hospital for injuries fairly often.
From what little we see of Hugo in OFF, he seems to be this. His voice grunting is a cough, and exploring The Room shows writing that mentions having to take pills. There's also all the brain and coma imagery in the battles, items, and game over screen, but that's going into WMG territory.
The quarians from Mass Effect are a species of Ill People. Since their forced exile from their homeworld 300 years before the events of the games, quarian immune systems (which were already weaker than those other species) have deteriorated to the point that all quarians must wear environmental suits at all times just so they don't die. Every quarian Shepard meets in the games—from Wrench Wench party member Tali'Zorah to badass marine Kal'Reegar to the valley girl complaining about her boyfriend on Illium—is one suit breach away from potentially deadly sickness. That said, they don't appreciate the stereotype:
"I'm not gonna die from an infection in the middle of a battle. That's just insulting!
The main characters of Narcissu are a terminally ill boy and girl. The prequel adds two more ill girls to the cast, one of them an 8-year-old orphan. And the third game... just say the whole series revolves around this trope.
The Guilty Gear franchise gives us a female example and a male one who's a subversion:
Female Example: Marina from the Ride the Lightning novel, a wheelchair-bound young lady who is the sister of Solaria, a woman who was horribly experimented on and turned into a Gear — which could have happened to Marina, too. She then accidentally drinks a drug named Vitae, which cures her disability but in fact is made of Gear cells, which start causing her massiveBody Horror. She barely survives thanks to Ky and others, and later is reunited with Solaria and given the protection they both need, but loses both of her legs and has to return to her wheelchair.
Male Example: Bedman, a comatose young boy piloting a sort of hybrid between a Mini-Mecha and a bed. He's been subjected to horrible experiments that allow him to trap people in nightmare worlds of his own creation. Subverted in that he is actually a villain - more exactly an agent of the Senate, the driving force behind Ky's appointment as a Puppet King, amongst other things.
Undertale's Neutral path explains that the Ambiguous Gender First Human was adopted by Asgore and Toriel after they fell into the Underground, but one day fell ill and died, after which their adoptive brother Asriel absorbed their SOUL in a fit of grief, took their body into the nearby human village, and was killed by the inhabitants who believed Asriel was responsible for their death. This lead to Asgore swearing revenge against humanity, a plan that disgusted Toriel so much she left the kingdom to live in the Ruins. Very tragic. Which makes it hit harder in the True Pacifist path, when you learn that they actually poisoned themself on purpose as a Thanatos Gambit. They told Asriel that the plan was to leave the barrier (which requires the amount of power in a human SOUL plus a monster SOUL), kill just six more humans, and return to shatter the barrier (which requires the amount of power in seven human SOULs), thereby freeing monsterkind. It's not made clear whether they were lying all along, or just got Drunk with Power, but when it came time to execute the plan, Chara wanted to exterminate the entire village, and possibly all of humanity. Asriel's willpower was able to stop them, but got him killed as he refused to fight back.
Calliope was a rather sickly child and Kratos went on a quest for Ambrosia to help her condition.
In God of War (PS4), Atreus suffers from some kind of debilitating condition that manifests as Incurable Cough of Death for most of his life. Mimir suggests that the illness is psychosomatic, a result of Atreus' struggle to reconcile his divine nature and emerging abilities.