Done, got Unicorn Rider, but don't have enough room for her. 
That's fine, you're early on yet so you'll run into that. Just keep playin and eventually you'll start wondering how the heck you'll EVER use all the space you have!
Also, Unicorn Rider is probably a more usable leader for early on than Avalon Drake due to her much lower cost(even evolved her cost is lower than Drake's UNEVOLVED cost), plus you can use her Leader Skill with anything you want of the right categories.
For now, try getting mostly light cards in your team with some fire ones to boost your RCV more(A Carbuncle will be good for that at the earliest stages) and see how that treats you. U.Rider won't take you into super late content as a leader, but she can help for now til you get more powerful stuff and the cost to use it.
This game will eat your children and then ask for a tip
This is disturbingly accurate, and why spending money on it is NOT ADVISABLE. Others do it, and that's fine, but realize it's the same as gambling; VERY real chances of getting useless things, especially as you progress and things that seemed strong early on show themselves to be rather unsuited for difficult content for any number of reasons.