Brutal Cathar
is just a very good soldier card-- good stats and acts as removal.
Kytheon, Hero of Akros
is fine, but much better if we actually get to flip the card. As this is a soldier with good stats (2/1) for just , this seems like including a single copy is fine.
Myrel, Shield of Argive has fantastic abilities. The only drawback is that she cannot be put directly onto the battlefield by Preeminent Captain. Indeed, that is the only reason there's only one copy of her in this deck.
Preeminent Captain is solid-- it can actually dump creatures directly to the battlefield, and it has first strike.
Recruitment Officer is the sort of card we don't mind seeing turn 1 or turn 2 because it is a 2/1 for , and we don't mind seeing it later in the game because it can allow us to dig for threats.
Resolute Reinforcements provides two creatures at instant speed. This allows us to rebuild quickly after a board wipe, get chump blockers out at instant speed, and generally allow our board to go wider faster.
Soldier of the Pantheon is a 2/1 for , so we can't really complain. Occasionally its abilities are relevant, so a fine card to have in a soldier deck.
Thalia's Lieutenant is just a fantastic card. Yes, I know, there are non-humans in the deck. Who cares? Most are humans, and look at some of the top decks in the format-- Selesnya Angels plays cards that aren't angels. It's fine-- most of our creatures are humans and this will build up creatures when it enters and get out of control as we play creatures.
Thalia, Guardian of Thraben is remarkably useful in most matchups. Sure, it makes Kayla's Reconstruction and Brave the Elements cost an extra , but that's a small price to pay to slow down Izzet Phoenix, UW Control, etc...
Valiant Veteran is the lord, giving our soldiers +1/+1. That's a solid reason to include the card, but the ability to put +1/+1 counters on our creatures later in the game from the graveyard is what makes this card good enough to power this tribe.