Isabella, is one of Charlie's ex-girlfriends and a satanist.
Isabella was one of Charlie's girlfriends whom was in a relationship with him purely for purposes of her covenant with a satanist cult. She at first has the usual casual sex relationship with Charlie, though Alan believed she had cursed him by giving him a stye (swollen left eye), though Charlie continues to see her despite this and the fact she tried to have a seance with Jake and the 'horned one'. Isabella take Charlie to see her thirteen 'sisters' and convinces him to join the covenant in exchange for sex with all of them. Charlie then awakens in his bed two days later with a pentagram drawn on his stomach. Apparently he and Isabella have been having sex for two days, trying to create a special prophecy child who would create terror in the world. Charlie says no, only to learn that if he doesn't, he will instill the wrath of a curse that will 'shrivel his manhood'. He was going to when Evelyn arrived and scared Isabella away after learning she's Charlie and Alan's mother. According to Evelyn, they knew each other from a pilates class, which they go along with.
In the Season 7 episode, "Gumby with a Pokey", she was seen with many of Charlie's other ex-girlfriends.
In the Season 9 premiere, Isabella is present at Charlie's funeral. Here she reveals that Charlie gave her herpes.