Betty Grof is Simon Petrikov's girlfriend. At the time that Simon put on the crown that eventually drives him to madness (turning him into the Ice King), she ran away from the power-mad Simon until a future Simon (who temporary had his wizard powers revoked causing him to age rapidly) opened a portal with the prospects of telling her goodbye. Instead, she leaps into the portal to try to bring Simon back, leading her to go mad with wizard power after collaborating with Magic Man. Her crazed-warped motivations to bring Simon back safe has more than caused turmoil within the world and the entire universe.
Lena Dunham voices Betty in her first three appearances, before being replaced (due to controversy of her autobiography) by Tom Herpich and later Felicia Day.
Lena Dunham, I personally don't care for her take. It sounds monotone and one note, and when she tries to emote, it doesn't feel natural. Reminds me of Marla Frumkin, frankly.
Tom Herpich plays Betty through a disguised voice filter, and he does pretty good. Nothing much to say but it's funny to hear that voice come out of Betty.
Felicia Day I feel plays the tragedy part most effectively. The scene she has in Elements and the finale are probably the most emotional I've gotten at her scenes, and you can really hear how far gone Betty's gotten with Felicia.