Much like Yuuki herself, Yuuki's Hyper changes every turn.
On odd-numbered turns, her Hyper Card is Angel Hand, which allows Yuuki to select a player, heal them to their maximum HP, and earn 10 stars for each HP recovered. Since this hyper allows Yuuki to select her target, this is best saved for when other players HP are low, especially if they are high HP characters like Marie Poppo or Arthur. Given this card's expensive cost of 30 stars, it's best to save it for when you can heal someone for more than 3 HP to get a net gain from its use.
On even-numbered turns, her Hyper Card is Devil Hand, which selects a random player, reduces their HP to 1, and gives Yuuki 10 starts for each HP lost. This hyper does not allow you to select a target, and is best used when playing against high Max HP characters while their current health is high. Due to the random effect of the hyper, it is hard to guarantee a positive outcome from the use of it, especially since you have to do more than 3 damage with the hyper to get a net gain.