While a flat % reduction on maint would certainly make it more balanced... I think leaving alone, and maybe even buffing it by 20% or something is a more interesting possibility. Drawback being instead of buffing all equipment by 100(or 120% if buffed), it only takes effect on one item slot (the first sub item slot i guess). This would reduce its value by even more than a 50% reduction in theory, but will less so in practice if your 3 items are not equal. I didnt play post much though so im not sure if there is anything like a machine god lucifer (which you can only hope of ever having one of until you ready finish all the post-game).
This also might nerf maint less in te early game too where the player only has 1 or two good/decent items assigned over 4-5 characters.
Okay, given that info, how's this, then?
Maintenance
Skill Point Cost: 3
Max Level: 5
Stat boosts and effects from equipment in the first sub item slot are boosted by (SLv * 30)%.
I hope that a maximum level effect boost of 150% to that one slot isn't considered too much...
Also, is there anything in that could be put in the main item slot that would be broken by increasing its effect? Because if not, then there's no reason for said skill to not affect that as well now is there?
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On a separate note, I had thought of how to actually help out Rinnosuke as a character: by giving him this...
Spells:
Force Slash
Single Target Attack Spell
Element: Physical
Formula: (100% ATT + 50% MAG) - 50% enemy DEF
MP cost: 5
Post gauge use: 50%
Leveling up reduces gauge usage by 1000 and MP cost by 1 per level(Lv2 = 60% and 4 MP, Lv3 = 70% and 3 MP, Lv4 = 80% 2 MP, Lv5 = 90% and 1 MP)
Flash Bomb
Single Target Attack Spell
Element: Mystic
Formula: (100% MAG + 50% ATT) - 50% enemy MND
MP cost: 5
Post gauge use: 50%
Leveling up reduces gauge usage by 1000 and MP cost by 1 per level(Lv2 = 60% and 4 MP, Lv3 = 70% and 3 MP, Lv4 = 80% 2 MP, Lv5 = 90% and 1 MP)
Reasoning: Rinnosuke possessing some attack spells wouldn't be a bad thing from my point of view, and these Spells, while weak as all get out, are at least marginally better in terms of formula than the Attack command from the start, although the MP cost of said spells make them a poor choice to even consider using in the early game. However, skill point investment does fix the MP cost problem, as well as speed them up considerably, to the point where he can at least use these spells as a means to delay his turn slightly while trying to deal damage. These spell, plus the "First Aid" and "Battle Command" spells that he already has, would make him a more well rounded character.
As a second point, I would also make it so that "First Aid" could cure all ailments other than debuffs when said spell is maxed out(Poison and Terror at the start, Lv2 cures Paralysis, Lv3 cures Silence, Lv4 cures Heavy, Lv5 gives the recipient 1 MP, essentially making it free for Rinnosuke to use on himself and let's him slightly restore another person's MP). The reason for not curing debuffs with "First Aid" is that he has "Battle Command" to deal with that problem.
Skills:
Given that he already fills up all six slots, I would do the following...
Fuse the money making skill and the increased item drop rate skill together, so that it takes up less space.
Fuse the attack debuff skill and the magic debuff skill together, for the same reason.
And now that there are two free slots, I would fill them with "Healthy and Lively", so that he can recover from ailments that do hit him faster, and as strange as this may sound, "Extra Attack", because him having a chance to do any of his moves repeatedly without additional MP or time loss would help him out.
What do you guys think of my little idea for improving the poor guy?