What it does
Roseli Berry only covers Fairy: an opponent lands a super-effective Fairy move on the holder, it gets halved, and the berry is eaten.
Fairy punishes Dark, Dragon, and Fighting — three of the highest-output groups in the game. Fairies are everywhere in the format (Flutter Mane, Sylveon, and the rest), and one Moonblast or spread Dazzling Gleam disciplines Dragons and Darks fast. A Dragon that eats a super-effective Fairy hit usually ends up in the red or dead. Roseli halves it so you can still move that turn.
Roseli stands out from the other resist berries for one reason: Fairy moves are so common in Champions that nearly every team packs a Fairy damage source, so the berry pays off more often than its siblings.
Same split from Focus Sash as always: Sash needs full HP and dies to chip and spread; Roseli works from any HP but only covers Fairy. If you know a Fairy move is coming, Roseli is the steadier pick.
Who runs it
The carriers are Darks, Dragons, and Fighters for whom Fairy is the killer.
Sableye (62%!) is the headline user. Dark/Ghost, with Fairy as its only real weakness — it's a slow, annoying disruptor (Will-O-Wisp, Fake Out, Quash) whose biggest fear is getting one-shot by Fairy. Roseli lets it tank that hit and keep being a nuisance.
Altaria (8%), Dragon/Flying, takes Fairy as a major weakness and badly wants the berry while it sets Tailwind or stalls as a Roost tank. The Dragons — Garchomp (2%), Kommo-o (2%), Dragapult (3%) — all carry a lethal Fairy weakness, and opponents keep a Fairy around specifically to remove them.
On the Fighting side, Gallade (2%), Conkeldurr (3%), and Machamp (3%) are Fairy-weak. They want to get in your face and attack, and Fairy is what checks them — Roseli gives them one clean swing.
How to use it
The rule: hold it for that one Fairy move.
With your berry-holding Dragon or Dark out and a Fairy attacker across from you lining up Moonblast, Play Rough, or spread Dazzling Gleam, eat the halved Fairy, stay up, keep the turn.
Watch Dazzling Gleam in doubles: it hits both opponents, so if both your mons are Fairy-weak and standing together, Roseli only saves the holder. Don't stack two Fairy weaknesses on the same side when building.
Sableye with Roseli has a bonus: it already runs on Prankster priority status moves, and the longer it lives the more annoying it gets — surviving the Fairy hit buys several more turns of disruption value.
Tips & strategy
The best use is on a disruptor like Sableye that opponents can "only really KO with Fairy." They click Fairy to clear the pest, Roseli halves it, it stays up, and it keeps clicking Will-O-Wisp and Fake Out — their Fairy move is wasted and your tempo holds. That's where the 62% usage comes from.
Don't give it to anything Fairy doesn't hit super-effectively. Roseli only halves when Fairy is super-effective; neutral or resisted, it does almost nothing.
Run the number first. Altaria, Dragon/Flying, is Fairy-weak but not especially bulky — a high-Sp.Atk Fairy clicking at full power can overkill so hard that even halved it still KOs. Then don't bank on the berry; pivot out early instead.