This is pretty much all I know of her, would have put it on the wiki but I HATE FANDOM 😡😡😡

but no jkjk, I just more didn't have an account until today so I can't throw the info there

Follow me if you think I'm cool or smth

Given her previous unfortunate interactions with Folly it makes sense as to why Melanie don't want anything to do with her.
Though it is possible to get Folly in after Melanie is in the elevator where she has a scared look on her seeing the recent updates on her wiki in fandom

Moving on to the more so interesting part (to me) is how she interact with Folly or the very least how she enters the elevator with Folly in it. If you get to a floor with Melanie can get in the elevator woth Folly there she would act like the elevator is full

This is great as a more controlled version of the fiz up drink though the randomness of her giving it makes it more so up to when she does it if anything (also while the effect on floors like the Minefield it still prevents the player from jumping making the effect useless there)

The other effect seems more random or at the very least once every couple floors so far is some kind of double jump. When she makes a beeping noise give gives a double jump effect (no idea if it's to a random person or to everyone though seems to be the former)

Moving on to what she provides, she seems to two buffs. One from throwing roses at her would give healing one that gives half of 1% (0.50%) of your total hp for 5 seconds. Without any items that raises your hp it would be 5hp everytime you throw roses at her

Afaik she just spawns at Mozelles Humble Castle with her running down the hall and stairs to the elevator. Her off points is with Turret, and Shop Space. She could probably also get off backrooms like the majority of npcs but that part remains unconfirmed