This is one of the best Signature allies in the game.
Drax has a very aggressive style, regardless of what actual "aspect" he is playing. This is going to result in a hefty amount of damage dealt to his identity card. Even if you can chump block and avoid any overkill damage, with retaliate damage as apart of his kit, there is incentive to allow Drax himself to take some of these shots.
This can obviously leave you in a bad spot, as 14 HP aren't going to last forever. Mantis' ability to heal identity cards for a damage to herself allows you to stretch that pool of HP further and stay in hero mode longer after you have built all your counters up. Not only that, as she is healing you for a 3 to 1 rate, using things like first aid on Mantis takes a card that heals for two on her into 6 healing on Drax.
Mantis is also not limited in only healing Drax. She can heal other players as well. So in Multiplayer, if Drax can get himself into a situation where he is getting tough tokens, either from cards like Hard Knocks, or from other players, those heals can get spread around no problem.
Finally, with a thwart of 2, that is always a option for a character with low thwart stat if an unwanted side scheme shows up, or if you desperately need to chip away at the main scheme. This gives her a little more versatility, in particular for the Aspects Drax naturally synergizes with: Aggression and Protection.
Definitely the Gold Standard when it comes to Signature allies!