Here's a Pokemon that I am going to mention: Inkay and Malamar.
Availability – Route 1 (Hau'oli Outskirts), 30% chance. Can also be found on Ancient Poni Path and Poni Wilds (20% chance in both).
Evolution – Level 30, but your 3DS/2DS has to be flipped down upon leveling up to make it work. It's a strange evolution method, admittedly.
Stats – Inkay's stats are basically your average unevolved Pokemon's stats. Malamar has pretty decent stats everywhere, expect Special Attack which is a little below average.
Typing – Dark/Psychic. Although it leaves it with but one immunity (Psychic), no resistances, and two weaknesses including a 4x, that's not the big kicker here.
Movepool – If caught at the beginning of the game, it'll have Reflect as soon as you catch it, although given its underwhelming Special Attack, its best STAB attack it'll get for a while is Foul Play, and even that uses the opponent's Attack stat, but that is very good early on. It can also learn Topsy-Turvy, which is amazing against the Totems (more on that in a bit). If caught at Poni Island, it'll have Psycho Cut all ready to go, and Payback if caught at Level 41 or 42, but you'll miss out on Topsy-Turvy for all the bosses ahead. And at Level 48 it gets Superpower, which is amazing if you have Contrary.
Major Battles – Despite its lack of good STAB moves for a while, Foul Play is very good early on and can help against Ilima, Totem Gumshoos, and Totem Marowak. But more importantly, Topsy-Turvy is an amazing move to use against the Totems, because it reverses all their stats from the Totem aura and turns them into stat drops, making them laughably easy if you can pull it off.
Other Notes – If it has Contrary and you're against an opponent that you think will try to lower your stats, don't be afraid to switch it in, because you're sure to get a helpful boost if they do. However, as mentioned, it can be a little hard to use with its lack of good attacks, although having Reflect at the beginning of the game can be a real lifesaver.
Also, I think you forgot to mention Hapu in the big battles that Popplio is great for.