Sephiroth's wing mechanic can trigger earlier than intended against the Pokémon Trainer due to quirks with down special in Super Smash Bros. Ultimate
Sephiroth receives significant boosts to his attributes and gains additional properties on certain attacks whenever his wing has sprouted in Super Smash Bros. Ultimate. The benefits brought on by the One-Winged Angel mechanic are triggered based on Sephiroth's current standing in the match.
If Sephiroth and the opposing character have equal stocks with one another, then the wing will make an appearance once Sephiroth accrues 80% damage. Of course, this means that Sephiroth will need to wait a little longer if he takes a stock lead. However, there's a certain scenario where the wing will sprout earlier than intended when facing off against the Pokémon Trainer.
As demonstrated by Lea (and explained in English by Ruben), the Pokémon Trainer's down special — the Pokémon Change — will directly impact the formula that determines when Sephiroth is due to get access to his boosts.
"On Pokémon Change frame 13, the game doesn't consider Pokémon Trainer as a player which alters the activation formula reducing the needed percent to trigger it on stock advantage," stated Ruben.
Back in 2019, we saw something similar happening with Joker and Arsene. Whenever the Pokémon Trainer used the Pokémon Change down special, Joker's Rebellion Gauge would receive free meter. Of course, this would later be fixed in a later patch.
More than likely, this issue will likely be addressed in a future update now that it's known. Until then, it might be better to not perform Pokémon Trainer's down special when behind on stocks and when Sephiroth has accrued 80% damage — unless you're fine with letting the other player have their One-Winged Angel form earlier than what's intended.
Sephiroth Winged mode activating earlier due to Pokemon Change explanation
— Ruben (@Ruben_dal) January 30, 2021
On Pokemon Change frame 13 the game doesn't consider Pokemon Trainer as a player which alters the activation formula reducing the needed % to trigger it on stock advantagehttps://t.co/HII6Udpwq8
