Creek and Nathan tend to appear as a pair in Fire Emblem: Fortune’s Weave, and that also ties in to how you recruit the redheaded cavalry unit. You basically can’t get one without the other, and it takes a bit of doing to ensure both make it into your Part 3 army.
Editor’s Note: There will be major spoilers for Fire Emblem: Fortune’s Weave and Nathan below.
Like Creek, you can’t recruit Nathan in Fire Emblem: Fortune’s Weave until you get to Part 2 of the story. However, you also need to ensure you help his ally and partner recover a valuable via a side quest in Part 1 first. If you do that, Creek joins after War Chapter 2: “Taking Agris Stronghold.”
The opportunity to get Nathan immediately follows in War Chapter 3: “Beyond the Salt Road.” He won’t immediately be on the map when the fight starts, but make sure Creek is one of the units you deploy. Once the fight starts, Nathan will show up in the first round of reinforcements. He also won’t wait for you to reach him to spring into action. He’ll immediately charge forward and head down the alleyways on the right side of the map.
You need to make sure Creek fights Nathan and is the one to land the final blow. I found Nathan tends to go down the alleyway on the right. Keep a healer like Dante right behind Creek, to heal him nearby, and someone with Physic like Tialla within range. Also, have Kalla’s Blessing enabled and equip Creek with a weapon that is strong against cavalry.
If Creek deals the final blow, a conversation will occur between the two. Nathan will leave the battlefield and, when you get back to the hub as the fourth chapter starts, he’ll be among your recruits. I shared a screenshot below showing exactly the place where Creek should be positioned in order to ensure he’s the one who faces Nathan in battle.

Like Creek, Nathan joins your army as a Bardinger after you recruit him in Fire Emblem: Fortune’s Weave. He’ll come with Guandao+2, and the automatic resupply also stocks him up with an Antitoxin, Concoction, Iron Spear+3, Short Spear+2, and Iron Sword+3. He’ll know the Personal Ability Headlong Rush, which grants him AS+3 and Hit+10 if he starts a fight. He also knew the Combat Arts Armored Blow, Tempest Spear, and Vanguard Spear when he joined my army.

Fire Emblem: Fortune’s Weave is available on the Switch 2.