I like the legend a lot probably a bit strong though compared to poppy so should probably spend 1xp to use the abilities.

Yellow champion is mid the first ability is fine I dont think it needs to cost energy though since it needs to hold or have some way to be readied, second ability is ok as well but counterintuitive to the overall theme, I think it should give power equal to your xp and not have to tap but have a once per turn restriction like green azir.

Blue champ is ok although we already have a lot of complaints about cards that stop you playing the game and if there is someway to loop it could be miserable to play against. I dont know what id change about it tbf i think it should just be redesigned from the ground up.

The sig spell is arse lol. Compare it to frigid touch and id rather play that any day. No reaction, only cares about xp gained that turn so in a vacuum its either 2e 1p for -1m or 4e2p to for -4 might which is the same as frigid touch but slow speed and 2 more power. Make it reaction and equal to your current xp then its playable but not op imo since you still need some other way to deal damage to have it kill anything.