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.

Warframe: Trinity's "Secret" Combo

Despite being a "healer" Warframe with all four powers geared to restoring the Health and Shields of her allies, the Trinity has one nasty combo that makes her a very efficient boss hunting machine (against some bosses anyway). It all has to do with the math of her first and second of her abilities, which is conveniently the sequence you need to remember to use her secret "attack".

Step 1: Cast Well of Life on Target.
Effect: Target will GAIN a health multiplier (x10).
Effect: Anyone that hits it will regain health.

Step 2: Wait for Well of Life timer to reach near 1 second.

Step 3: Cast Energy Vampire on same Target (that should still have Well of Life)
Effect: Deals pulse damage based on enemy's remaining amount of health and shields.
Effect: Anyone that hits it will regain Energy.

Step 4: Well of Life expires.
Effect: Target will lose the health multiplier, returning to original values.
Effect: Energy Vampire will still do damage based on the original, boosted values.

Result: Target very owned.

Using this I soloed Ambulas, the Corpus MOA boss on Pluto to farm up some Excaliber parts even though my Trinity was still under leveled. Alas, the bosses that require soft spots to be shot will probably be immune to this and on the many normal mobs it's generally faster to kill them with your weapons.

On a side note, that's an interesting design decision Digital Extremes made there. If you didn't pick Excaliber as your "starting" frame the only way to get it later on is to fight your way from Mercury outwards, beating all the bosses to unlock more planets and satellites until you reach Pluto which is home to some pretty high level Corpus.

I can't think of any other game off hand where something you are offered right at the start can be so difficult to get later on. :P