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Michelle Flammel from Monsterful, She's in love with the fluffy Rag doll Jacob.
Jacob is also one in the first chapter, he's seen stalking Sapphire.
Damian from Zoophobia harbors an unhealthy infatuation with Kayla, who has a boyfriend, and is incredibly creeped out by Damian.
El Goonish Shive's Justin has one — Melissa, who has stalked him since their one date in High School... which was where he told her he was gay. Apparently, that has not dissuaded her. Since they actually grew up together, she counts as an Unlucky Childhood Friend to boot...
The Gene Mage in Terinu fits this trope for Terinu's race, given a tendency to refer to them as his "sweet limbed Ferin", with an extra helping of creepiness considering he deliberately created them to look cute and perpetually somewhere in their mid-teens.
Eastwood in Exterminatus Now was (Or maybe still is) one of these, as he had his ex-girlfriend's new boyfriend put to death by a squad of special forces that are normally used to combat the paranormal. The fact said new boyfriend was actually a heretic only came to light after he was shot to death on the lavatory. Eastwood also seems to be rather overprotective of what she still sends him, such as attacking his associates with an axe after they ate some of the biscuits she sent him.
Autumn became a client of Suicide for Hire because the victim of her stalking publicly humiliated her after she went too far.
She tried to murder his new girlfriend.
Namir Deiter: Cedric Van Helgen towards Tipper Namir. His unwillingness to take "Go away" for an answer culminated in his getting punched out by Tipper's fiancee, Charles.
Cassie from The Wotch, with the classic symptoms of obsessively following Robin around, trying to slip him love potions, and other doings of questionable ethics. A recent run-in with her own Stalker with a Crush seems to have made her see her wrongdoings.
Yet another one that seems to have finally gotten somewhere.
Jason was this to Anne at the very beginning of the series, with Robin even noting that "In some cultures, this is considered stalking). Jason drops his crush - and stalking - pretty quickly when he sees Anne levitate a pencil though.
Sinfest neatly encapsulates the trope here, with Fuchsia the succubus's crush on Criminy the shy little nerd. The devil finds this vastly irritating, as it makes her act less evil than she's supposed to be.
Gunnerkrigg Court has Diego. Good lord, Diego. It got to the point where he programmed all the robots he built to worship Jeanne as a sort of Christ figure. And when he gets rejected, he recommends her for a Human Sacrifice. Hell hath no fury like a lonely, big nosed inventor.
At least he didn't tell the robots about that last part, so they still see Jeanne in a positive light. Better than nothing.
Wiglaf & Mordred - Brat. As soon as she lays eyes on Mordred marrying him becomes her goal in life.
Vriska is this to Tavros, although in her case, it's less "spontaneous, unashamed affection" than "I am attracted to him, and that makes me feel uncomfortable...so I have to hurt him for making me feel this way." Evidenced on this page, where she both relentlessly bullies him and tries to make him say that she's pretty.
Persona 3 FTW turns Aigis into one of these for the main character. She was canonically into him in Persona 3 itself, but here it gets taken to "has a habit of humping his leg" levels.
In this strip of Loserz. "I spend all of my time with them, I swear I'll be with them forever *sob*, I hide in their bushes..."
In PreTeena, Johnson Pinecone teeters on the verge of this in his pursuit of Jeri Keene, always managing to be there and knowing from observation what she likes.
Oh Young Gon from Cheese In The Trap is like this to every girl he fancies. It mostly involves giving them unwanted attention, and secretly taking photos of them at school. These girls tend to get him to back off by publicly humiliating him, as only an over-the-top rejection can convince him he's been rejected at all. However Hong Sul happened to show him an ounce of kindness when his latest rejection turned him into a social pariah. He has since been convinced that she secretly loves him, and taken his stalking tendencies to a new level: following her home, sending her unwanted gifts and texts, and trying to behave as though he actually were her boyfriend. Not even Sul threatening to call the police on him can convince Young Gon that she's anything more than shy. All of his recent actions have apparently been part of a plan to get Sul and Jung to break up, and to convince her that he's not a creepy stalker and is actually good boyfriend material. The fact that he has such a plan does not help his case. Nor does following her around the library surreptitiously trying to take her picture. And he still won't take no for answer. Some of this might sound tame compared to other examples, but the fact that everything is played out realistically makes his actions genuinely frightening.