It's actually 2 less forma than stated here, you can just swap V from top left corner to exilus slot. You save another forma if your mods are simply not leveled all the way. So it's 1 - 2 forma build. The rest of the setup ... is not :D.

Warframe mods

You are probably wondering what the hell is this? Where are all the stats we surely need. And the thing is, these ARE all the stats we need. Her strength scaling is cope(mei), the difference in damage between this and max strength is negligible. The healing multi from her 3 is cope(mei), that is NOT going to save your life, it's just used ... because it's just there ... sometimes it actually heals you (cope) and once in a while it gives you VERY long invulnerability (cope(mei)). We get some strength anyway from Arcane Impetus, alongside another boost to efficiency, which is what I really want from this arcane. Duration is not needed, because the duration of her CC is generous 20 seconds by default and nothing else scales with it. Range is good, but you probably don't even need this much, because the range on her 4 is again, pretty decent by default. You could probably drop [Augur Reach] for some other mod, but you just don't need anything. For this reason, you can simply build defense and energy economy. Arcane Blessing feels well aligned with the idea of Koumei - getting progressively stronger, the longer you are in a mission - and creates more of a threshold for health tanking. Once it builds up a bit, you are highly unlikely to die, thanks to the other sources of healing in this setup, that are not her cope(mei) 3rd ability, which is supposed to do that, but just ... doesn't really :D.

PS: Yes, the exilus mod is unironically just there to make me wall latch longer. Use whichever one of the wall latch mods you want or choose something different. You can also just not unlock the exilus slot to begin with.

Archon shards

1 yellow energy filled on spawn for comfort - optional
2 yellow casting speed shards - she needs the casting speed, but you can swap out her augment for Natural Talent instead
2 green +2 health on Toxin status damage - optional, it's not the only source of healing, but feels more comfortable

Helminth

Sickening Pulse - this helps you apply status even more effectively. It's not actually mandatory for this setup, because it's a hybrid between weapon platform (mostly melee) and caster, not a pure caster DPS setup. Not to mention Sickening Pulse refuses to bump the best DPS statuses anyway. It's just cool to finally use this Helminth ability and have it do something good :D. What I like about it is that I don't need to build huge strength and good duration to make it do some good, unlike the usual Roar's and Nourish's.

Weapons

[Daikyu Prime]

I run this because it just looks great with her as fashion accessory and because of [Amalgam Daikyu Target Acquired], which is important for this setup. I use the Acuity setup, because it's the only way to make it do any real damage and it can fit [Amalgam Serration] for extra Sprint Speed. But if you think you will just pop heads of enemies CC'd by her 4, then ... well :D. The way they wiggle makes it more challenging than getting a decree that doesn't suck. If you want to save resources on this build, you could probably just hold it as a stick, just put the amalgam mods in it and never use it. Build - https://overframe.gg/build/890564/

[Dual Coda Torxica]

This is a flex slot, this build doesn't require any specific secondary weapon. I just use this one, because it's good and I like it. You just need to make sure you can effectively kill enemies with this weapon. I don't even use the ranged weapons much, but sometimes you want to use it to complete one of her decree challenges or to lower CD of her 2, if you are running her augment. I think it's beneficial for this weapon to NOT be incarnon. When you use it rarely, like me, the incarnon charging is just annoying. Build - https://overframe.gg/build/890565/

[Syam]

To leverage the healing from [Daikyu]'s amalgam mod, we want a nikana. I chose this bow and nikana setup, to stick to Koumei's japanese theme. Keep in mind, that it must be one-handed nikana, two-handed [Nikana]'s don't work with this augment (Azothane for example). It doesn't have to be this one, but this one is cool and has a unique gimmick, the damage is OK-ish and it's easy to get. Everything should work as intended with any nikana of your choice, we just want the healing and japanese vibes. Build - https://overframe.gg/build/890577/

Companion

For melee lifesteal health tanking without innate status immunity source and PSF, you will need a [Wyrm]. It doesn't have to be fully invested [Wyrm] like mine, but you do want [Manifold Bond] and [Negate]. [Negate] is your lifeline and [Manifold Bond] can shorten it's cooldown. It prevents DoT effects from killing you when you can't lifesteal and prevents knockback, which can make a life or death difference. I'm running Tazicor with Synth Deconstruct (* note below ... :D) to fuel Arcane Blessing, alongside some utility mods.

Wyrm build - https://overframe.gg/build/890826/
Tazicor build - https://overframe.gg/build/890832/

For higher level content - I mean, something noticeably above T5 SP bounties and such, some SP endless or whatever it is you are doing over there :D. You can get more damage out of your [Wyrm] by using [Contagious Bond] and [Verglas]. Using it in regular SP is plain annoying, because it's harder to build combo and complete some of the challenges.

Wyrm build - https://overframe.gg/build/890589/
Verglas build - https://overframe.gg/build/890592/

How to play the videogame

[Koumei] isn't particularly complicated to play. Load into a mission, use your 2 to get a challenge and use your 3 to cope(mei) it does something. You can hold 2 to abandon challenge you don't feel like completing. Getting kills while latched in public lobby running SP bounty is not a thing, that happens in real life :D. You could just try to quickly latch and cast 4 to damage everything and even if the enemies are exploded by your team immediately after (they will be), it should count for your challenge. Killing enemies that are close is easy, because you will use melee a lot. Killing enemies that are far, can be done by casting your 4 (and optionally subsumed 1) and shooting your secondary. Killing enemies afflicted by status is free. Then hold W and and melee button and enjoy the mayhem. Use her 4 frequently to CC enemies and spread status. Use Sickening Pulse immediately following your 4 if you have it, to significantly improve the status application. The damage is decent and the CC is quite good. Picking up health orbs increases your max health. Damaging enemies with toxin status (that just naturally happens) heals you a bit if you have the green archon shards. Beating enemies with your fancy samurai stick, heals you a lot. Combination of these factors turns you into a pretty good tank, with very good CC, solid damage and infinite status application, getting progressively stronger as you are getting some actual useful decrees ... hopefully ... cope(mei).

Final note

Yes, I did mention the phrase "weapon platform" in [Koumei] build guide unironically. And honestly, what did you think she is? Read the decrees :D. She is not a pure weapon platform, she is not good enough for that. But she is certainly not AoE nuke caster either, because she is also not good enough for that. She is a hybrid by design and that's how I build her. I try to use her abilities in tandem with my weapons (mostly melee, because I'm a smash monkey), as imho intended and when the halves combine, she is pretty good. I top damage in T5 SP bounties on regular basis, while being functionally unkillable (I just facetanked Legacyte and acolyte in T5 SP bounty some 15 minutes ago), which means she can comfortably handle 99.9% of relevant content in the game with this loadout.

You could say: "haha, carried by weapons, cope" and that would be cute. Because in most fast missions I don't ever shoot the gun and anyone who claims [Syam] on it's own can carry anything anywhere, is coping harder than [Koumei] mains on Reddit :D.

You could say: "haha, carried by [Contagious Bond], cope" and that would be even cuter. [Wyrm] usually doesn't even get to kill a guy before I do. Confiscate his [Verglas] and you won't notice a difference. It's just synergistic addition I made, because why not. If you intended to have the companion play the game for you, you would hide behind dupe [Nautilus] with [Fired Up] and Summoner's Wrath, which are absolutely massive performance increases for actual [Contagious Bond] carry. Update: um, yeah, I don't really get to say this cheeky BS anymore. After further testing, it felt like Contagious [Wyrm] is pulling way too much weight and gimps my playstyle by killing enemies before I get to build up combo. I will own up to it, by leaving this here. What I said before was true, when I somehow DID build my combo and started to outperform the [Wyrm] during earlier testing :D. That being said I changed my recommendation above. [Tazicor] is better for low level content (yes, I mean T5 SP bounties and such), because it benefits you without ruining your day by killstealing. You can bring Contagious / [Verglas] to high level SP endless if you feel like it. I will append YouTube video to this guide, where I literally realised this in real time :D

The build is basically a combination of many parts that contribute, without any one of them pulling significantly ahead, to solo carry the rest. But now I really need to go, to enjoy the copium ... :D