The thing is, if he's just 'left behind' and told 'hold that door' - he's a standard guardsman from the NPC's section if you want to work out a fight longhand. Since he's an NPC on his own against other NPCs, the GM can just arbitrarily decide himself and it makes no odds to the game.
At the moment, a comrade can
- Shoot at the same target as you
- Charge the same target as you
- Perform a task of challenging (+0) or easier whilst you protect him
Other simple tasks you might want him to do - I stand by 'he's an extension of you and shouldn't do things by himself'
- Drag you to cover - I like 'rescue' as you stated, but you're not a comrade - I'd suggest a half move. The average human, once you strap guard body armour on them, is juuuust outside the easy lifting capacity for a S3 T3 guardsman, so a half move feels right - half an action to move himself, hald action to move you.
- Watch your back - he's also a second pair of eyes. Possibly grant you a second reaction (at a cost of him doing anything?). In a melee, this would represent him parrying for you (potentially) either against your main opponent or against someone else trying to get involved, rather than helping attack your target.
- Look out, sir! - a rather malevolent use for it (probably a talent to buy, might cost corruption?) - voluntarily redirect a hit onto your comrade (either he heroically dives into the way to protect you or you…ahem…yeah. Equivalent of whats-her-face from Rogue Trader who's supposed to be especially callous with her men)
I'd leave covering fire as the sarge's order.