It’s only good for ATK-scaling units, especially ones that already have plenty of CRIT stats or lack external ATK buffs.
Passive + Weapon Details
Additionally, Isshin’s passive is called Isshin Art Clarity, and it has the following effects:
When a normal/charged/plunging attack hits an opponent, it triggers a Hewing Gale. This deals AOE damage equal to 180% of the wielder’s ATK. This also increases ATK by 15% for 8s. This can be triggered once every 8s.
This is a pretty easy passive to trigger, thus making it even better for units that need a lot of ATK. However, ATK isn’t the only stat a character needs – instead, a mix of other stats like Energy Recharge and CRIT is generally recommended. Plus ATK is often easily saturated thanks to the multiple sources of ATK buffs available in-game. Because of this, Isshin is easily outperformed by a lot of other weapons with better secondary stats or passives. For instance, Ayaka is a unit that benefits a lot from ATK, but Amenoma Kageuchi – a craftable sword – is still better than Isshin for her. This is because Isshin has a bit less ATK but a significantly better passive. This is also true for other units like Ayato, Keqing, and Melt Kaeya who would benefit a lot more from Lion’s Roar – but they still work with Isshin. Still, Isshin is a great alternative for most ATK weapons that you don’t have.
Best Character Pairings
Here’s a cheat sheet for Kagotsurube Isshin’s synergies with all current playable sword users.
Good means that the Isshin works well with the character, but it’s not among their best 4-star options. Decent means that it works on the character, but there are other better options (usually other 4-star gacha or craftable weapons). Not recommended means that it doesn’t synergize well with the character – or there are significantly better options.