In Full-Moon this character is very similar to F-Kohaku. She keeps all the same normals and her 236 specials. This includes her 5C, it's partial charge, and full charge. What F-Kohaku loses makes F-KohaMech a gimmick team that will probably only work once against someone who's never seen the matchup before.
Here's what she loses from F-Kohaku:
Molotovs
Magical Amber command dashes (including her ender)
Plants (hence most of her Oki)
Command grab
Counters
Kohaku Drug-On Install (163214C)
This obviously hurts a lot. Kohaku is stripped to bear bones and loses almost her entire vortex (plant oki); not to mention her all too useful counters which shutdown the enemy's projectile game hardcore.
Here's what she gains over F-Kohaku:
Assists
Air Assists
22-Series assists
623 Machine Gun Moveset
A different command grab.
Maid's special Arc Drive (214214C in max)
BnB
(2A) 5B (2B) 4B 2C 3C j.bc (sd)j.bc airthrow
Ground Hit Confirm -> 623C Dash 5C Aerial
Corner
(5B) (2B) 5C 623C 5C j.bc (sd)j.bc airthrow
(5B) (2B) 5C 236C 5C j.bc (sd)j.bc airthrow (If Mech-Hisui is not in position)
Remember you do lose that all important ender.
Full Blockstring
Of course, staggering 5B/2B/4B/5C is your best bet for scoring a hit, but F-KohaMech can use 100% meter (623C) to reset her string, something F-Kohaku cannot do. It's good for lockdown.
(2AA) 5B 2B 4B 5{C} 236A EX623C Dash 2B...Reapply
If you think you can get away with a jumpin (or not being reversal'd/poked), you can do...
(2AA) 5B 2B 5C IADj.C 5B 4B 2[C] 236A 623C Dash 2B...Reapply
The dash is to cover jumpouts and counter hit anything. You have enough time to do it, the timing is fairly easy to get down after a little bit in training mode. The full string does have gaps but if you manage the whole thing it's great for guardcrush. Does about half a bar. Obviously you don't want to do it against some characters (Like Ciel who will flashkick you out of your IAD j.C or ExHiero after your 236a). The nice thing is if you're smart you'll know where the holes are and can rotate accordingly.
The IAD is your option point, from there you can just j.c to continue with the above blockstring, feint a charge into command grab, or go into a low 4B, 2A, or just straightout charge j.C.
The 623C is bunkerable, but you can reapply in different ways to punish your opponent if they pick up on this. You may also want to save the reapply for if you're in max (as C/F can EX guard it) or if you're fighting halfmoon. Otherwise it might eat up meter you'd want to spend on her...
IH Combo
(2AAA) (5B) (5C) 236A 5E 2C 236A 41236C
2C 236A 5E 2C 236A 41236C
Roughly 4-6k and circuit break. With the loss of magical amber, you can't IH Mixup for a 4way, but her j.C crosses up and you can use some IH options with that. Feel free to post if you make a good mixup with it. 623C in Blockstring is also good to IH.
Command Grab Tech Trap
63214C will command grab allowing the opponent to tech. In the corner as you launch them you need to do
63214C 22A (or 22B for short characters/neko's)
This will create a tech trap. If they tech forward you can 236B if they attack of 5C if they don't for counter hit. If they tech backwards they will be caught by 22A/22B and forced to block, allowing you to reapply pressure. If they tech neutral you have a VERY small gap to 5C them into aerial combo (and even if you fuckup they'll be back to blocking in the corner and you can reapply pressure. No matter which way they go, you should be able to hit them, counter hit them, or reapply pressure. Usually you can confirm into a combo.
To perform this correctly, you need to command grab them, buffer 22 while Mech is summoning her extra mech, and hit A or B when the hit occurs. The timing is somewhat strict. You need to really watch for how they tech. If they don't you get a free OTG string, 2AA 5B 5C, and another tech trap.
Opponent's Wake-Up setups
22B 4B
This feints that 22B is an overhead and you catch them low with 4B. You can confirm into a combo. I would like to note 22A blows for anything other than the command grab setup, so I suggest using 22B.
22C Dash Grab
One fucking godlike Bazooka. It will link into your grab. Even reduced this will do a grab for about 3k. Works at most ranges. Once they figure it out, you can do mix it up with 2C or 4B.
Ground Artillery
This is a strategy I use to control the ground. Full screen you can use Mech's 22C to send a projectile across the screen. You can then use 5[C] to send Kohaku's sword (which will hit crouching characters) to help get in, and link and/or use your Edge Waves (236A/236B) to control a lot of zone around you. You even have enough time (if you can get on the other side of your opponent), to 5[C] to send projectiles across the ground each way. This strat Loses to Air-Ground fullscreen (such as F-WLen's Ice, or Ciel's Swords) and is very weak to crossup. Usually you can get away with it fullscreen.
Assists
Overall these a very weak, but like with any assist character in any game, you can use them sneakily. 214A should be low, but it isn't, and 214B should only be used to hit your opponent in the air. Except you can't even get counter hit damage off it because she hit's them twice. 214C works as an anti-air, but you already have 2B, j.b and 236B anyway so it's really only useful in the air. You can hold 214C to launch multiple projectiles if you want to. I"m not completely sure, but I don't feel like any her assist hits are plus frames. If they are you have too much recovery to do much about it.
Trades
Just wanted to make on final note on trades with her 22-assists. Trading is a pretty important tool for F-KohaMech's gameplay. F-KohaMech's assists are weird in that they have a fast startup but horrible recovery. Meaning, Kohaku will still be in recovery for her 22-move series while her 22A/22B will still go off. Thus, you can actually use 22B randomly and while your opponent combos you (if they don't stop) they can get hit from the assist and stunned (bad) while you run it back with your own combo. Nice trade for like 1.5k on you and 4.2k against them.
Overall F-Koha&Mech is really gimmicky but her setups are easy and heavy damaging. Regardless, her normals are just as strong and these two can hold their own in a fight against characters with low mobility have some interesting tech-trap and guard crush options against characters with a weak defense. If her assists were a bit better she might be a competitively playable B+-tier character.
Thanks to Ehrik for reviewing this guide, improving on the tech trap, and listing important notes about her 623C.