How do the TF2 pets work?

Solution 1:

After some more testing and spending a bit more mana, I finally figured it out:

1. The text when you mouse over the attack rating for the TF2 minipets are wrong. They should state heal or repair, but it says base damage.
2. Despite this, their heal/repair rating is actually based their base damage rating. Its just that the medic's base damage starts out so small (5) that it takes several upgrades to bump it up to a noticeable improvement in heal rate.
3. The medic is pretty bad as he heals really slow. There's an in game pet (looks like a fairy) that heals much faster (if they have the attack speed boost), and gives a more coverage of skills.
4. The engineer's ability is not unique either. There is a pet called Abaddon that also heals nearby defenses, and again, he does it at a much faster rate, making the engineer obsolete.
5. The pyro attack is pretty unique as well, it attacks fast and is aoe, but its hard to aim at enemies as he'll often shoot above them. His attack is very similar, but less range, to the unique huntress flamethrower you get from the summit boss.

The heavy, unfortunately, is not that great as there are better pets with more skill boosts that do more damage in game.

I have not figured out a way to get a replacement TF2 pet if you accidentally sell one (I didn't want to risk losing mine and have no way of getting it back). If anyone can find out more about this that'd be appreciated.

Solution 2:

The engineer I know heals in the area, and seems to be able to heal more than one tower at a time. However, this DOES cost you mana. I've found that playing huntress, he often depletes my mana pretty quickly, leaving me unable to use my Piercing Strike ability as I like to spam it.

I can't comment about the medic and whether or not his ability costs mana. But I would imagine the ability costs a low amount of mana to sustain, and is probably only active if your health is not full.