noun
one's favourite member of an idol group
Slang used in idol-fan communities. 自担 is a contraction of 自分が担当するメンバー (the member one is in charge of supporting).
私の自担は、グループのセンターです。
My favourite member is the center of the group.
自担の誕生日にファンアートを描いた。
I drew fan art for my favourite member's birthday.