Gloomyrain

Built-in Rhianna and Luciella

Gloomyrain's S1 has the extremely powerful ability to give her an extra turn. There are no conditions or strings attached: every time she uses her S1, she has a chance to get an extra turn. This even works on Counterattacks or Dual Attacks! Pair her with Dual Attack-focused allies like Tamarinne, Lilias, or Ras and she can amass quite a lot of turn speed.

Pivots off Debuffs

When afflicted by a debuff, Gloomyrain's Atk and Spd increase. The Spd increase means that she may be able to outrun the next enemy in the queue and get a turn before them, at which point she can use her S3 to slap whatever debuff she was inflicted with onto the entire enemy team instead. This is a particularly great counter to AOE Defense Break or Unbuffable units like Basar and Tywin.

Deceptively High DPS

Gloomyrain's skill damage multipliers aren't anything to write home about, so it's easy to think her damage potential isn't that high. However, with her high base Atk, passive Atk buff, and chance to give herself extra turns, she can actually deal quite a lot of damage in a short span of time.

Immune to Sleep, Stun, and Provoke

Sleep, Stun, and Provoke are devastating ailments in Arena and Guild Wars that can singlehandedly win a fight. Gloomyrain is immune to all of them, meaning she is virtually impossible to disable. (Silence does prevent her S3, but it also triggers her passive and turns her S1 into a small atom bomb, so calling this "disabling" is debatable at best.)

Speed Devotion Aura

As of December 2019, Gloomyrain is the only Mage with Spd on her Devotion Aura. As a 3★ hero (even if she is a Moonlight hero), it's not as hard to pull multiple copies of her as it is of some other Spd-imprint heroes, giving her allies a much-appreciated turn speed buff just by existing.

Requires Investment

For a 3★ hero, Gloomyrain requires a lot of investment before her potential becomes apparent. In particular, she needs to Skill Enhance her S1 and S2 to max before she really gets up and rolling. As a 3★ hero, at least she uses Stigma instead of Molagora for these enhancements, but the Catalyst investment is still huge.

Fragile, But Needs Aggro

Gloomyrain has to be afflicted by ailments to unlock her full potential, which means she has to take hits from the enemy. However, her HP and Def are quite poor, so taking hits from the enemy can be pretty risky.

Unreliable

Gloomyrain is very dependent on RNG and on the exact composition and actions of her enemies. If her S1 doesn't proc extra turns, if the enemy doesn't use the right debuffs, if she accidentally resists a debuff when you wanted it to hit her... there are many ways that using Gloomyrain can go wrong.