Warframe: Saryn and Trinity

The double-locked frames. Not only do you have to RNG the bosses that drop their parts, you need to first farm keys to fight said bosses! To be fair, Mesa falls into this category too. :P

Saryn

Collected from Kela De Thaym on Sedna (after you've done enough of her arenas) Saryn is all about spreading her toxic spores which is her first ability. Shooting said spores or killing the spore carrier spreads the disease to other nearby enemies and so forth. She can also shed her skin and make it explode with toxin, toxify her melee strikes to always hit spores and lastly basically fart which infects enemies with a viral status (HP takes more damage) plus high direct damage to enemies affected by spores.

While she shines in endless type missions where you have big rooms full of respawning bad guys, her contagion is less useful in missions that aren't that type as often victims will die without being able to pass on their spores forcing her to start the disease from scratch. She's also less effective if grouped with a competent team as they'll probably kill the disease chain before it can propagate too far. Decent frame, but situational.

Trinity

Collected from the Ambulas boss facility on Pluto (that you can only get to by killing other Ambulases on Pluto, aim for the legs!) Trinity is designed as the healer frame, and her number four power is arguably the best at it: an on demand shield and health recharge to allies in range provided you have energy for it. This does mean to be effective you'll be watching the team stat bars more than any other player.

Alas her other powers are lack luster: immobilize a single enemy and make them immune to damage but anyone that attacks it heals themselves, mark an enemy to pulse renewal energy to nearby allies, and link to an enemy so that any damage done to Trinity goes to them instead. Ideally you'd want to use these on tough eximus enemies but due to their over shield mechanics, you can't until you hurt them severely. This means the link and energy pulse are near useless as foes should be getting killed, not left wandering about for a temporary buff.

The healing thing is also self defeating for as you shoot that single target whom you can't kill, everything else will out damage your healing - meaning you should focus on killing? Now that more and more frames have self healing (thank you Archon shards) Trinity is really becoming more and more irrelevant.