Everyone remembers Lozenge Magus. It was Misaki's first starter, it was in her trial deck, it was reprinted numerous times. Now think of a Lozenge Magus but better, even though strictly speaking they do the exact same thing. This is Sunny Smile Angel.
Whenever Sunny Smile Angel boosts, whether the attack succeeds or not, you have to put her back into the deck. Seems kinda lame, but remember that Smile Angel is a heal trigger. This means that you can drive check a heal trigger, heal your damage, then next turn call it for a boost and put it back into the deck, potentially getting her again at some point. Want to start the process earlier? Smile Angel can also be a starting vanguard, so you save the space normally reserved for a starter and can instead put in a higher grade card.
Strategies and game play:
Fantastic, but how is this any different from Lozenge Magus? It's because Smile Angel is a part of Angel Feather. If you damage check your Smile Angel, a return to the deck is only a Nociel away. Angel Feather switches cards from the damage zone easily, so a heal trigger revealed from the damage check can still be recycled into your deck. This makes Angel Feather a very good clan for defensive, attrition-based players.
Combos with other cards:
As mentioned above, Nociels can take your damage-checked Smile Angel and put her into your hand (where you can either guard with her or play her and use her skill again). Another useful unit is Circular Saw, Kiriel. With her on-ride skill, you can call a damage-checked Smile Angel, even without a Nociel. My personal favorite Smile Angel combo is with Cosmo Healer, Ergodiel. If Ergodiel launches a successful attack, its user can CB2 for her Persona Blast, discarding a spare copy of herself from the hand to recover a point of damage. Now imagine all of the times you were placing Smile Angel back into your deck, where you can Twin Drive into them. You now have a recovering vanguard, who statistically will get you your Smile Angel at least once per game and can potentially heal two or three damage from a successful attack (one from her Persona, and one to two from potentially twin-driving into the Smile Angels you keep recycling).
Ways to counteract it:
While Sunny Smile Angel can be a starting vanguard, it's far from the only options. You're far more likely to want to begin your fight with a starter whose skill will directly benefit your playstyle, like Crimson Heart for a Metatron build, Miracle Feather Nurse for an Ergodiel build, etc. And while putting a heal trigger back into the deck seems like a good idea, think about it as losing 10k shield that you could be using to block attacks. Blocking attacks means not needing to hope for heal triggers as much, even a heal trigger as useful as this one. Additionally, you end the turn with an empty RG space, so you'll need to spend more potential guard playing a booster to fill the empty space. It's a good option to have, but you might be better off filling up your field once and not wasting your best guardians on gambits that may not work.
Recyclable heal trigger that can be pulled out of the damage zone via the clan's central mechanic, enabling more guarding or more chances to heal.
More guarding OR more chances to heal. If "more guarding" is chosen, it can be overcome with a lot of powerful attacks. If "more chances to heal" is chosen, it can be overcome by exploiting the temporarily lower amount of guards. Additionally, Smile Angel is not a "Celestial", so it can't be used fully in some decks (such as Zerachiel who can't Especial CB her and Penuel who can't superior call her).
Unlike Lozenge Magus, Sunny Smile Angel only comes in one variety: a common from Breaker of Limits.
Artwork and aesthetics:
A heal trigger actually looking like someone medically trained to heal. Other than that, there's not much to say about the card art.
Ten points for being a heal trigger (mandatory in most if not all decks), with -2 for the things it can make you do to best utilize her skill (such as not using her to guard), -1 for not being in the subclan (though admittedly she debuted before the subclan) and +1 for making great use of the clan gimmick. Sunny Smile Angel is worth 8 out of 10.