Sometimes characters will use a Heart Symbol ♥ in their speech bubbles, or sometimes dead-centre in their mouth. This is supposed to indicate their tone and show that they are energetic or a kawaiiko; that, or they are squeeing like mad about something. Sometimes they're just talking in a sing-song manner, sometimes they're doing it in a flirtatious manner, and sometimes they're reacting to something that happens to feel really good.
However, sometimes it is done sarcastically by a much less enthusiastic character, or used in an inappropriate sentence in order to add an element of extra creepiness to a spooky person.
Sometimes it is even Lampshaded when other characters mention the hearts, usually with a line like, "Don't say that with a ♥!".
Related to Lucky Charms Title, and more distantly to Wingding Eyes. A lighter variant is to use a music note to connote sing-song or facetiousness.
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Anime and Manga ♥
Marmalade Boy has Miki saying that she can't believe her parents said the sentence "We're getting divorced! ♥" with a ♥.
Urahara in Bleach is a frequent example of the sarcastic approach.
Yachiru uses them, too, because she's basically a Cheerful Child. Hideously powerful, but cheerful nonetheless.
Chizuru uses the flirty squee type of hearts when she gushes over Orihime.
Nemu's vocalizations are littered with these during the Hueco Mundo arc when Mayuri does... something to revive her from a very disturbing near-death experience.
Gin's cheerful 'Bye-Bye!♥' after kicking the gang out of Soul Society and cutting off the gatekeeper's arm in the blink of an eye.
Rangiku Matsumoto has hearts associated with her as well. There's also an amusing sketch in Bleach Official Bootleg of Hisagi asking for feedback on an new advice column he's set-up. Rangiku's feedback shows she thinks it's silly, stupid, hard to read, unreliable and simply not gripping. She breaks his heart, and she does it with hearts and flowers doodled all over the signature and comments boxes.
Tsukoyomi uses these as she describes mutilation, torture and general mass murder and carnage. Definitely on the disturbing side. She drools too. Oh and also when she's talking about Setsuna and Konoka. SoYeah.
Konoka herself throws them around with great vigor, especially when she's healing or talking about Setsuna.
An early chapter of Tsubasa Reservoir Chronicle has Kurogane (the gruff Ninja) say "Kurogane is very happy ♥!" If this seems out of character, it's actually the Team Pet Mokona imitating Kurogane's voice.
Similarly, the Satan-icHIM from Power Puff Girls, but unlike the Millennium Earl's speech, which could be seen as "grandfatherly," HIM is just a bizarre amalgamation of the devil, a lobster and Dr. FrankNFurter.
Dakki from Houshin Engi does this pretty much every time she speaks. Additionally, her sister Kibi does this with stars.
Lucia in Mermaid Melody Pichi Pichi Pitch ends sentences with these when she's going on about her boyfriend or after she attacks with a "Love Shower Pitch! ♥". Hanon takes this to a whole new level, putting them after almost all her sentences and sometimes in the middle of words.
Kankuro from Naruto gets the creepy version; he does this just before his puppet snaps every bone in his Sound Ninja opponent's body.
Karin often uses them when flirting with Sasuke (In the Viz manga, this is accompanied by a fairly cutesy-looking font when she does it in Volume 38). Ino also uses them when preparing to use Mind Transfer Jutsu to get Sakura's answers during the Chunin Exam.
Also, Ino does this several times in volume 8.
Kakashi does this a few times early on: "You... Pass! ♥"
Noah does this on occasion. It's decidedly not cute.
In ef, Renji's Cloudcuckoolander mom Sumire often appends hearts and musical notes to the ends of her speech.
Oddly enough, this crops up often in Detective Conan. Quickly gets disturbing when in an early appearance, it's dragged out for a Ho Yay joke.
Conan: You do have a reason for making us come and stay here, right? Hattori: No, I don't... I just wanted to be with you~♥ (Conan aims his watch stun-gun at Hattori.) Hattori: Ah! Don't shoot! I'm only joking!
Butterflies Flowers: Domoto insists that Choko address him as "Director Domoto ♥" when on the job.
Narumi from Gakuen Aliceis this trope, in the "squeeing like mad" or "extra-flirty" sense. (He has 28 hearts in the first chapter alone.) Misaki has Lampshaded this at least once. Also, Rui Amane Says It With Stars. Presumably he's just that sparkly.
In an episode of Mirumo de Pon!, Kaede gives Matsutake a coffee cup and includes a note, which his servant reads to him, including saying "heart mark" at the end.
Battle Royale is an intensely psychological manga with lots of deception and desperate attempts at heartwarming, so it is no surprise that its Manga is littered with this.
Memorably, the Big Bad Yonemi Kamon's speech is littered with these, all the while he's talking about rape, murder and the imminent bloodshed involved in The Programme.
In Saiyuki, Hakkai will do this. Usually it's to indicate that he's in full-on Team Mom mode, and treating the other main characters like unruly children, but he also did the creepy version once when threatening to break a minor villain's neck.
When Futaba tells Puni Puni Poemi she wants to see her transformation. Yes, that's an anime, but they made an Art Shift for this scene.
The most prolific user of hearts in Ah! My Goddess is Hild, chief demon and the ruler of Hell.
In the Wedding Peach manga when a Bride is possessed by a love hating devil she gets aggravated by anything to do with love, and even tells Wedding Peach that the ♡s Peach is speaking with are pissing her off.
In the Maiden RoseRosa Alba doujinshis, Enrico ends every sentence with a heart when he's in charm mode.
Interestingly, done by Natsu in Fairy Tail, upon making Lucy do his signature move thanks to Kain Hikaru's voodoo doll.
Natsu: Fire dragon's... Iron Fist ♥
Mirajane does this before proceeding to curbstomp Kamika who was trying to kill her with poison.
Mirajane: Poison? ...That's merely a dessert for a demon like me ♥
In Sangatsu no Lion, Hina and her friend briefly speak this way while fangirling over Takahashi during his first appearance. Variants occasionally appear with stars or flowers to signify joy or happiness.
Also, the sculptor, Carlov, speaks with hearts quite often.
There's another (rather odd) example in the Four Swords manga, where Red Link utters a ♥ after Gufuu grabs him and Blue and ties them together.
In Twilight Princess, Link's fangirls in Hyrule Castle Town have hearts in their dialogue box whenever Link talks to them. In fact, while they're swooning over him, they'll drop three actual hearts to replenish Link's health.
Fangirls: EEEEEEK! It's HIM! ♥♥♥
Deneb from Ogre Battle ends nearly all of her sentences with a heart. It's so much a part of her character that the heart being in a different font in the sequel Tactics Ogre kind of ruins the effect.
Millenia in Grandia II uses these sometimes after a few of her more... playful lines of dialogue.
This Paper Mario games tend to pepper their text with hearts, especially the original. This LPer has some fun with the tendency.
As the page quote indicates, this get used somewhat frequently in the PC-98 Touhou games (except the first, which kinda lacked dialogue sequences). It disappeared by the first of the Windows games. And yes, the three primary "offenders" (Yuka, Mima, and Marisa) were all varying degrees of crazy, so it tended to follow a particularly violent or mean statement (again, see the page quote).
The protagonist's mother in Dra Koi has a little heart in her mouth while in the middle of a smoker's cough.
In Apollo Justice: Ace Attorney, Examining a letter from Detective Ema Skye reveals that she signed her name with a heart. Genki Girl Trucy, a magician, responds by saying that she always signs her name with a diamond. She suggests that Apollo Justice should sign his name as "Apollo = Justice."