1. So far in the demo, there are only 7 knights the last you can only recruit it if you beat the Dragon Knight in Cycle 8 Each knight has different base stats: some are more focused on strength, others on agility, and some have balanced stats. Then there are their preferences.
Each knight likes different things. For example, the second knight likes duels, hunting, and retrieving relics, but don't send him to retrieve the relic stolen by the magpie because he'll die since he has no agility and will fall from a height. They also have dislikes, like the first knight who doesn't like killing or the third who hates the dark and anything to do with commoners.
2. Regarding armor repair, depending on the quest and your knight's stats, their armor may be damaged. The more damaged it is, the higher the chance of failing the quest or dying, so don't let the damage get too low, or it will cost you dearly.
3. So far, only the pirate mission is mandatory because if you don't send help, the port will be destroyed, but it's for the best since none of your knights are strong enough to defeat the pirates and you'll have to go back in time.