Those aren't buffs, they are just what attribute a ninja is tagged as. If you hover your mouse over them, it will display what it is called if you are unsure. An example would be Lightning Mains, Root of Warrior which affects 3 sword users. So Lightning Main and 2 other random sword users in your team will gain the buff of immunity and +100% resistance. When you are in the Ninja tab, either Team or Train, you can sort by those attributes to find ninjas that would be affected by certain skills. So if you sort by swordsman, it will show you all ninjas that would be buffed by that skill. Look at a ninja's skills, then go find other ninjas that will pair well with those skills.

If you have questions about specific characters, feel free to ask for more clarification.