I'll tell you the image I had in mind.
Ten years ago, Belleros goes out to play (he'd be 15, so, for an Eladrin, I guess 9-10 years old) and finds Mask, abandoned, alone, wounded, that's up to you. Since Belleros is a "hyper" kid, and the only kid in his large, uptight, noble family mansion, he takes him/her/it home, and causes a ruckus to convince his parents to "adopt" his new friend.
Pretty simple, but all the complications of a noble eladrin and a doppleganger growing up like siblings/friends in the Feywild (so in Eladrin society) can be built in.
What do you say?