Much obliged for the thorough reply. I'm doing fine with the battles so far, because I'm willing to grind a bit, I abuse the bersek mechanic to speed things up considerably. I'm gonna go with the Seijiro route, because honestly he's hotter than everyone else... but who should I pick as my second companion for first time? I'm thinking Mia, because she's got First Aid. I would like to go with Seijiro and Kippei and get some hot sandwich action right off the bat, but not if skipping endings will harm my experience.
New Game plus runs speed things up after you know what to do. Kippei is very good for your early runs, because having expanded inventory is just too good. Later runs still make him useful, because you'll be doing less item management with your storage box, which means less running back and forth. Everyone else has unique items that can be replaced, but do provide convenience.
If you're going based on items and not stats or phobia, then I recommend taking Yachika along if you're not taking Kippei. She makes your runs faster thanks to her hairpin. You won't need to search for small keys with her around, and there are locks only she can open. Usually, these lead to more items.
Mia's first aid becomes less useful the quicker you end fights since she needs a character to be afflicted by either poison, paralysis, or injury for her to use her first aid kit. Rikana's ability is the weakest in the long run, because you'll eventually know all the secrets, lore, and enemy attributes you want in the future after using her dictionary or the memos.