Flowey isn't a character Frisk can meet in UMH. Rather, he's one of the inner-demons Papyrus may mention to Frisk.
Appearance
Papyrus describes Flowey as looking like one of the Golden Flowers that he enjoys planting in UMH's Courtyard, though there's a few slight differences. Flowey has piercing red eyes, and appears to be in the process of possibly wilting.
Relationships
Papyrus
Flowey, being one of Papyrus' demons, despises said skeleton and will do anything he can to cause him pain and torment. Though harboring more demons than just Flowey, Papyrus is more inclined to do what Flowey suggests as he's the most powerful.
Sans
Flowey repeatedly expresses his hatred for Sans. On occasion, Flowey will try and trick Papyrus into hurting/attempting to hurt Sans. He later uses these encounters against the two, making them fight and grow more distant.
Alphys
Flowey doesn't like Alphys, and likes to have Papyrus pick on her when he gets the chance.
Undyne
Flowey hates Undyne as she does Sans, though knowing Undyne can be aggressive he doesn't tend to go after her.
~More Information coming Soon~