noun, noun-modifying (irregular)
yandere; person with an unhealthy romantic obsession
Slang term from anime/manga fandom; describes a character who is initially loving but becomes mentally unstable and possessive, sometimes violently so. Portmanteau of 病む (yamu, 'to be sick') and デレデレ (deredere, 'lovestruck').
She's a yandere, so she has a slightly scary side.
The heroine in that anime is a typical yandere character.