Koumei is kinda my main even though I'm fairly new and she's really fun to use.
There's basically two way (which can be combined) to play her: as a weapons platform, hoping (and cycling, remember you can reject a challenge that sucks) to get good Decrees to supercharge your weapons and murder people or use S1 and S4 as a Caster frame to murder people with status effects, since IIRC those scale with enemy levels, so there will never be a time were throwing a bunch of random statuses at enemies will be a bad idea, her S4 also immobilizes people and basically hit everyone you can see, giving you ample time to stab them. It's a great nuke even without investment.
Her unique gun looks cool but isn't very good. Her unique polearm, the Amanata isn't best in class but it's good enough and very funny, because suddenly you will roll all sixes and just supermurder people for the next 30 swings. I use it a lot.
Since both her offensive skills (and a bunch of Decrees) do statuses and she can randomly do extra status effects with her weapon attacks do to her passive, she obviously likes things that procc off non-specific status effects, a lot. You might never know if you will manage to procc specifically a Heat status on someone, but if you throw out a bunch of bullshit at them, they WILL get a bunch of statuses, just not a specific one.
Since all your stuff deals with status effects, you can forget Ability Strength and just go for efficiency, range, duration, things like that.
S1 also serves a secondary use as a short-lived trap, since the threads do statuses on people that walk into them, so you can lock down a corridor very effectively, if not for very long.
Starting a fight by throwing a S1 or S4 (depending on the toughness of the fight) is always a good idea.
Koumei is extremely powerful in longer missions and free-roam since all your Decree buffs are permanent and stack, so you can get pretty silly if things go on long enough. On the other hand, you might just get random bullshit that doesn't actually help you. Or you might roll "double melee damage" on your first one and be set for the mission. No way to know.
Or you might get a triple jump
She does have a bit of a hard time against bosses since status effects are more limited against them, but if you know you're up against a boss, you can always take a detour and try to farm random mooks for a couple of extra Decrees. If you get good ones (there's multiple ones that are basically"double damage"), you can take down bosses very effectively.
Her S3 is a extremely efficient survival skill outside of really tough things that can one-shot you, and also leans Koumei is a lot more resistant to toxin status than you might think, it allows you to be more cavalier with her health and stay in fights after your shields are gone longer than you otherwise could.
She's not the most powerful frame on the market, but she is very very fun. And very funny.
Tailoring her weapon loadout for making it easier to get Decrees is a good idea too, I use the bow from New War for all the headshot and "kills over 20m away" stuff, and a Kompressa Prime for all the things like "kills while airborne" or " kills while sliding", since you can just cover the enemy in bubbles and then have ample time to jump, slide, wall latch, etc, while they explode.
Neither weapon is optimal for actually benefitting from all the Decrees, which really likes high fire rate, big magazine guns. But they make *getting* the Decrees very easy which to me is a worthwhile tradeoff. And Nataruk hitting twice is never a bad thing.
