Teysa, Envoy of the Ghosts is sorta a germaphob, and hates it when creatures damage you, it freaks her out. So whenever a creature does damage to you she kills it and sends in a Spirit friend to help you out.
Attended Knight, Doomed Traveler, Bloodline Keeper, Skirsdag High Priest, Midnight Haunting, Captain's Call, and Lingering Souls will help make a wall of Living Soldiers and Dead Zombies that will attempt to help stop things from damaging you.
Protecting your army is a number of spells that will either destroy your opponent's creatures Nekrataal, Shriekmaw, Annihilate, Malicious Affliction, Austere Command, Necromantic Selection, and Planar Cleansing. Or spells that will protect you and slow your opponent down Fiend Hunter, Intangible Virtue, Oblivion Ring, Arrest, Sphere of Safety, and Rootborn Defenses.
Darien, King of Kjeldor also hates it when creatures damage you too, so if any do he will call his Soldiers in to f@ck shit up and help you.
Xathrid Necromancer on the other hand doesn't like your Human friends or himself dying, so if they do he gets pissed and summons a Zombie to avenge you.
Overseer of the Damned really likes it when your opponent's creatures die, especially if they have damaged you a lot, so when they do die you get a Zombie. YAY
Captain of the Watch and Hero of Bladehold are always good to have around. They don't care if things damage you but they like to fight so yeah.
Geist-Honored Monk really just likes being around a lot of people, so much so when she arrives, she brings two Spirits along with her. Guess she has separation issues or something.