You may receive a cutscene that involves someone wishing to write a story about your adventures with your fellow. If you receive this cutscene, travel to Ru'Lude Gardens and speak to Ajahkeem near the Rendezvous Point to quickly finish the quest. This quest is to enable the use of 'Fellow Points', and has nothing to do with the level cap quest. Talk to Luto again to receive the proper cutscene to trigger Clash of the Comrades.
If you bring Reraise and die outside of aggro range, you can get up and heal. Your NPC does not regen HP.
Unlike the Maat fight, your fellow will engage you as soon as you are within range. If you come within range, your fellow will not wait for you to make the first move.
But, like the Maat fight, your fellow will give up at approximately a similar HP percentage.
Your Fellow's Level/Jobs
Your fellow behaves as a character with a main job level of at least 74 and a support job level of 37. This is an uncapped fight, so many advise to play it safe by fighting as a higher level character for an easy win.
Based on the discoveries made here: Clash of the Comrades/Fellow Job, one can deduce that the fellow's job and subjob are based on the player's current job&subjob: