Ratboy was a runaway teen who lived in the sewers because of his grotesque, rat-like looks.
Patrick was born with an unusual appearance. He looked like a rat, which prompted unfriendly kids to call him Ratboy. Tired of being teased, Patrick moved to the sewers, looking for seclusion. There, he met with "Gotham's secret shame" — giant man-eating rats, and befriended them.
He hoarded unwanted items and discarded property, proclaiming himself a "Pack Rat". He also developed hybrid flowers that grew without photosynthesis. His interest in the neglected and unappreciated was eventually expanded to people. He observed them from below, particularly those who he thought to be neglected like him. Then, he kidnapped them and offered them shelter from the ungrateful world. However, Patrick's kindness quickly turned to murderous anger when his hostages revolted against him. Patrick, then more assertive in his own habitat, killed those who would mock him by feeding them to his rats.
Encounter with Batman
At some point, he turned his attentions to Dana Tan. He started stalking her, leaving flowers with the note saying "I'll be there for you". When Dana was waiting for Terry McGinnis at Rhino's Chili, Patrick sent a couple of his hound rats to abduct her. He showed Dana his habitat and hobbies. Unimpressed, she tried to trick Patrick into taking her to the surface for some Chili. Patrick didn't want to risk losing her, so he went by himself. He barged into Rhino's Chili and was spotted by Batman, who then overheard him mentioning Dana.
Patrick returned to his place just on time to save Dana from being attacked by a pack of rats. Dana quickly grew impatient with her captivity. Patrick mistook her frustration for mockery and set his rats on her, at which time Batman stepped in and fought off the rodent army. In the brawl, a burning hockey stick made contact with dumped toxic waste, blowing everything up and likely killing Patrick as well, though no body was found.
Powers and abilities
Patrick didn't have any special powers or abilities, but for some unknown reason, he could communicate with rats, and therefore, command them.
Patrick returns in the comic Batman Beyond Vol 2 #17, "A Sinking Ship", where he is shown to have survived the explosion, though it left him with burns over most of his body. He comes back looking for revenge on Dana. This issue and other promotional books give Ratboy's full name as "Patrick Fitz", though his last name was never revealed on television.