Monster Hunter Stories 2: Wings of Ruin is an excellent JRPG that improves on its predecessor in every way imaginable, and even adds a brand new mechanic: the Battle Buddy. Battle Buddies are allies that form a party with the protagonist. If they're a Rider Buddy then they'll fight alongside their own Monstie, which increases attack and support options. Hunter Buddies fight alone but make up for their lack of partners with superior stats and lack of attack attributes.
However, as with any JRPG, there are some allies who are simply better than others in terms of combat effectiveness. Their effectiveness can be judged based on the overall synergy between them and their Monstie if applicable, how well their weapon complements their specialties, and how much does their AI help or hinder them during battle.
6 Alwin
He isn't a bad character, perse, but he falls victim to "only being good for extra damage output in a battle." He suffers under the mediocre AI, as he often falls into the trap of choosing a single attack type without accounting for the enemy's own strengths and weaknesses.
Not to mention that his weapon choice, the bow and arrow, doesn't exactly grant him any particular advantages and in fact causes him to trip up even more, as his AI can never decide on the appropriate time to charge his weapon.
In fact, his regular weapon and Tension skills tend to be expensive, so his Kinship gauge fills much slower. This means that he can't suppress the enemy as effectively as other Buddies. So while he's not necessarily a bad Buddy to ally with, there's no particular reason to use him with other, much better options around.
Though Alwin's Monstie does have a pretty cool design and his bow attacks look impressive, so players who use him for style points alone are completely valid.
5 Kayna
She's your first ally and the one who trains you in combat basics, the attack mechanics, and how to effectively pair with your Monstie. Kayna wields a short sword and shield, and fights with her Monstie Avmar.
She's a purely offensive fighter with a battle style that favors quick strikes and while her AI will occasionally fail her, it isn't to the level of Alwin. However, her biggest drawback is that she's essentially a less effective version of Cheval.
Similar to Cheval, her weapon allows her to quickly fill her Kinship bar, as her Tension and normal weapon skill costs are so low. This makes her a great beginning Battle Buddy, as she can lockdown her foes and prevent them from attacking quite often.
However, once Cheval is available, there's little reason to continue using her in combat. Unless you're accounting for her weapon being fire-based, which can be helpful at times.
4 Cheval
Cheval returns from the original Stories, and is a solid ally for your team. His main specialty is wearing down the opponent with skills that inflict status ailments such as poison, which is already incredibly useful.
He uses a sword and shield weapon, which at first seems rather unassuming, but his true potential with said weapon becomes far more apparent later on.
He and his Monstie, Rathi, are able to lockdown opponents, meaning that he prevents them from attacking as much as possible while dealing solid and consistent damage. His weapon choice amplifies this because his Tension usage and general weapon skills are cheaper than other Battle Buddies, so he can perform them much more often.
His Kinship, therefore, tends to fill faster and he can essentially spam them, skipping enemy turns outright when he does. When a Kinship skill is used against more powerful enemies, they will simply not use their most powerful attacks. Ergo, the damage output against your own party is significantly reduced.
3 Anivia
As soon as you see an ally with a hunting horn, you know intrinsically that they will be one of the best characters to fight alongside. At the very least she's easily the best Rider partner to use. This is absolutely unquestionable knowledge that any experienced Monster Hunter fan has, and Monster Hunter Stories 2 reenforces that incredibly well with Anivia.
Anivia, like others on this list, first made her appearance in the original Monster Hunter Stories and returns to aid the protagonist in the second installment along with her Monstie Frostfang. She wields the hunting horn, meaning that her specialty is buffing and healing allies.
These skills are always useful in a JRPG but especially worth their weight in gold in Stories 2, a game in which your own healing capabilities are extremely limited. While her attack strength isn't amazing, her support capabilities make her an incredibly valuable Battle Buddy, especially since they completely defy the normally terrible AI that plagues the others.
2 Kyle
Kyle is one of two Hunter Buddies and, despite his rocky start and possibly off-putting personality, he's one of the best ones to work with. Unlike Riders, Hunters fight alone and as a result, are stronger with a much higher HP pool.
On top of that, their attacks and skills don't have a particular attribute attached to them, so they can attack indiscriminately without chancing an enemy's counterattack. They will rarely be one-shotted by powerful attacks, reducing the number of turns that normally would be needed to revive and heal a Rider Battle Buddy.
Naturally, Kyle has all of these benefits, making him an excellent addition to your party, especially when fighting some of the strongest foes in the game. The only downside to Kyle specifically are the same issues that plague Alwin: namely that because he uses a bow and arrow, his AI has a difficult time discerning when to charge and when to attack normally.
Luckily, he has multi-target strikes that are great for sweeping enemy parties.
1 Reverto
Another veteran from the original Monster Hunter Stories, Reverto is a lazy yet skilled Hunter who regularly fights legendary monsters. Which is to say that if this man is in your party, he will carry you to victory. As a greatsword user, he has none of the drawbacks from Kyle's weapon and all of the benefits of a Hunter.
His biggest strength in battle is in fact, his strength. He is unequivocally the most powerful Battle Buddy in the game, with a damage output that easily outstrips even the protagonist. So even if you decide to literally do nothing in combat, he'll just go ahead and kill the monster himself.
If you plan on tackling any of the legendary monsters in this game, Reverto is an absolute must as an ally.