Fell!Calibri

Fell!Calibri is the UNDERFELL counterpart of Cali. He made his debut when the RG stooges (including Belle) get stuck in the FELL timeline, in which he fights them and delivers them to the Brig, consequently serving as one of the main antagonists of the arc.

Personality
Calibri is an obnoxious monster who has an constant urge to flirt with everyone, mainly to purposely make people feel uncomfortable and piss them off. His moral compass, like most other FELL monsters, is completely twisted and insane. Cali is shown to also like joking around frequently, constantly wanting to be entertained and amused.

Oswald and Tinero
Tinero and Oswald are the closest thing Cali has to friends. He seems to somewhat care for them, but gets frustrated with them.

Papyrus
Cali has respect for him, and even took inspiration from his battle body for his own outfit. He is loyal to him and follows any order he gives.

ST!Belle
Cali finds Belle annoying, but that does not turn him away from flirting with her. He only flirts with her to annoy her and ST!Cali.

Trivia

 * Cali often ends his sentences with a "Ha!" or a "Haha!"
 * He has an obnoxious laugh.