Ronin are masterless samurai who start as allies in Act 1 and become a regular enemy from Act 2. They fight like Jin, parrying and countering, so each one is a proper duel even though they come in small groups.
They use a stance close to Jin's Stone Stance, cancel attacks, dodge and parry, and punish you with counter combos. Before an unblockable they sheathe their blade, like duel opponents. In a Standoff they creep in carefully to hide their strike.
No stance is a hard counter, but Stone Stance is the most reliable. Better still, avoid swordplay: arrows, Kunai and bombs all work because ronin only know the blade.
Look for roaming groups, pillagers by a crate of loot, diggers in fields and pairs hunting a bear. Each group gives a minor Legend boost, except the bear hunters.
From Act 2 the ronin are your enemies because Ryuzo and his Straw Hats side with the Mongols. Ronin keep roaming Toyotama after Ryuzo falls at the end of Act 2, but no longer as Mongol allies.
Bandits
Bandits are the island's common criminals, attacking villagers, Mongols, ronin and Jin alike. They are a secondary enemy in Izuhara during Act 1, poorly armed and predictable, but reckless enough to hurt you.
Eagle Tribe
The Eagle Tribe is the Mongol force that has taken Iki Island, led by the shaman Ankhsar Khatun, called the Eagle. They are the main enemy of the Iki Island expansion, and their purple-uniformed troops bring units you never see on Tsushima.
Mongol Archers
Mongol Archers are the bowmen at the back of every Mongol group. They are fragile up close but chip away at you from off-screen, so kill them early or close the distance fast.
Mongol Eagle Bearers
Mongol Eagle Bearers are archers or swordsmen paired with a trained eagle that circles overhead and spots you. They fight like normal soldiers, but the bird makes sneaking up on their group much harder.