Trinity

Trinity is great for players who prefer a supportive role. Warframes with healing technology are rare making Trinity a great equalizer when the odds are stacked against the Tenno.

Acquisition
The Trinity Warframe can be purchased for 225. The blueprint can be purchased for 25,000.

Component blueprints drop from Councilor Vay Hek (Everest, Earth).

Cosmetics
Trinity can be equipped with an Aura helm.

The helmet itself, besides modifying looks, increases power duration while adversely impacting health, forcing a more supportive role.

Ability Overview
(All data is prior Update 7.  Due to buyable heal items, the abilities of Trinity are rather trivialized unless used skillfully.  A buff for Trinity was proposed by the Devs in their Dev Livestream regarding Update 7.)

Just as the ingame loading text says, Trinity plays a largely supportive role preferably staying back from the front lines. To emphasize the fact, it may be important to note that without a supercharge, Trinity's unmodded shields cap out at a beastly 100. But to facilitate that, Trinity's abilities possess very long to infinite range and Trinity packs a lot of health, as seen in the Upgrade Progression Tree. As a result of the shared long range of her abilities, it is advised for a Trinity player to pack around the Snipetron or the Latron as opposed to a Strun or a Gorgon to take advantage of said qualities. Due to the nature of Trinity's primary abilities, requiring allies to hit a tagged target for a certain boost, Trinity plays best against single strong enemies or bosses where her buffs can last longer and possibly affect more teammates, as any teammate that attacks the tagged target reaps his/her own benefits without sharing. In other words, the more resilient the enemy, the more effective Trinity's abilities will generally be.

Well of Life
Trinity's first ability uses 25 energy to create a Well of Life on a certain enemy, granting health to any teammates, including the Trinity, that attack it. Upon doing so, the target will light up the vicinity with a bright green glow similiar to a bright healing wave emited by Healing Ancients before retaining a bright green aura about his 3D model. The main goal of this ability is to, of course, heal; therefore a good Trinity player should be pressing the squad key (default 'Z') at regular intervals to keep check on her teammates. Despite healing teammates, Well of Life is significant in keeping the Trinity player herself alive. As said prior, Trinity is more of a health tank than a shield tank. It can be relatively sure that a Trinity player deep in combat will spam Well of Life simply because her shields drop like a stone in heated combat and her health pool is, despite the only thing keeping he alive, much larger and is protected by innate armor or armor mods. The range is extremely long, (while unconfirmed, I remember being able to tag any enemy practically in my line of sight, regardless of distance.  Confirmation appreciated.)

Energy Vampire
For 50 energy, Trinity can cast an Energy Vampire on any single enemy. Despite sporting quite the dramatic casting animation, an afflicted enemy will experience much the same glowing sequence/aura as Well of Life, except in light blue. Allies that attack the tagged target gain energy, and it is possible for the Trinity to near or completely reimburse him/herself for the energy costs, or at higher levels, even gain a net energy "profit" especially when fighting bosses. When fighting bosses, this skill is absolutely invaluable because it not only keeps your allies using their devastating abilities, but assuming you can score a great deal of hits yourself, it also keeps you in your strongest position (healer/support) in the fight for a longer period as well. Like with Well of Life, the range is extremely long, stretching at least half the length of a Grineer Asteroid Base boss room at rank 1 (perhaps farther...)

Link
In a bind, Trinity can perform a short deal of tanking herself with Link. For 75 energy, Trinity can throw a damage link between herself and an enemy (throwing a link between teammates has so far been inconclusive, confirmation appreciated.)  Almost a buffed version of Rhino's Iron Skin, Trinity becomes temporarily invincible, transfering all damage to a tagged enemy. Any status effects such as knockback, poison, or the like (except probably the Grineer Seeker nervos.. gotta love those things) will be nullified on Trinity and instead transfered to the target. And while Trinity's first two abilities end when the target is killed, Link remains active for its full duration (albeit short as is) even if the target is downed. The trinity remains invincible and unafflicted by status effects, but as the original target has been killed, nothing is transfered. Against bosses, this can be extremly valuable; Energy Vampire can return power more than Link+Energy Vampire. You can maintain Invincible status. Its range is also very long, though presumably shorter than Well of Life and Energy Vampire (confirmation needed?)

Blessing
Trinity's crown jewel, Blessing, heals all allies health and shields while applying temporary invincibility for 100 energy. It is debatable whether Trinity is worth the supercharge to obtain Blessing however, as player items like Team heal do a great job at healing a limited amount of team health. However, Blessing is not limited by the healing limits of said player items; a Blessing can heal a 1 to a 600 instantly while at the same time regenerating shields and applying invincibility. In a boss fight or when surrounded by regular mobs, I have yet to think of a time when an invincible bullet-repelling, claw-shunting, poison-proof suit was a bad idea. But perhaps its strongest quality is its range; while unconfirmed at the moment, playthroughs suggest that Blessing possesses an infinite range; wherever the teammates are, if a quick Z press or panic in the chat logs indicate a breaking loose of hell, players can receive a heal and buff anywhere (though not nearly anytime due to the energy cost.)

Mod Slot Polarity
Trinity has the following slot polarity:
 * 4 Warframe Ability slots (Scratch)
 * 2 Defensive slots (D)
 * 4 Neutral slots (non polarized)

See Mods - Polarity.