Choosing main character is so big deal? If so, which would you recommend for newbie with no prior experience in game?

---Choose Lightning Main, why? .. You'll know the reason later when you gain knowledge of this game.

How one can change looks of it's character (is this P2W feature?)?

---It is one of the Features that this game have, it is not fully p2w because Clothing are basically free if you have enough patience on getting its frags for free per event cycle. (take note that the clothing is on rotation so 1 cloth might not appear the next cycle and make you wait for another 1 to 3 months)

Is there any character one cannot acquire without paying, if so, lack of them will make experience underwhelming against P2W?

---All the characters here in this game is again, basically obtainable for free, and again, it depends on your Patience. (For this questions, it is same with Clothing, you can piece a ninja from getting free frags from events, other method is by saving coupons and not spending for useless stuffs, spending cps also everytime we have a good fuku rebate

Also, I'm not going to spend a single coin on the game, so F2P for me.

---I can say that im f2p cuz i never spent a single coin, but you can be good in this game by planning ahead, as well as by choosing right decisions .. In short, Think before you Click.

.. Thanks :)