Trinity mainly plays a support role, preferably staying back from the front lines. A Trinity player should stay around long range weapons as opposed to close ones, to take advantage of her abilities. Due to the nature of Trinity's primary abilities, which requires 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. Any teammate that attacks the tagged target gains their own benefits without sharing with the rest of the team. In other words, Trinity's abilities are more effective against more resilient enemies.
Well of Life
Trinity's first ability uses 25 energy to create a Well of Life on an enemy, granting health to any teammates that attack it, yourself included. Upon doing so, the target will light up the vicinity with a bright green glow. The main goal of this ability is to heal. A good Trinity player should be pressing the squad key (default 'Z') at regular intervals to keep check on her teammates. In addition to healing teammates, Well of Life is also good at keeping Trinity alive.
Trinity players tend to use Well of Life often because Trinity's health and shields are capable of depleting quickly as she only has an armor base of 15. The Well of Life range is extremely long but not infinite.
Before [Update 9.8], Trinity's Well of Life was not used as much as her other abilities, due to the small amount of life available per cast and the short lifespans of enemies. When used, it was cast on bosses and heavy units. As of [Update 9.8] Well of Life's duration is buffed to last ten times longer.
For 50 energy, Trinity can cast Energy Vampire on a single enemy. An affected enemy will experience a glowing blue aura, similar to the one created by Well of Life. Allies that attack the affected target gain energy, and it is possible for Trinity to regain the costs of Energy Vampire. At higher levels, Trinity is able to regain more energy than the cost of Energy Vampire.
To maximize the effectiveness of this ability, it is recommended to keep Trinity's ability duration as low as possible (i.e. using Fleeting Expertise and not using Continuity), since this shortens the time between the energy bursts.
When fighting bosses, this skill is valuable because the energy gain allows your allies to keep using their devastating abilities, and it allows Trinity to heal and support for a longer period as well. Similar to Well of Life, the range is extremely long, stretching at least half the length of a Grineer Asteroid Base boss room at rank 1, if not further.
[Update 9.8] Energy Vampire was edited to radiate energy to nearby players every few seconds, while stunning and damaging the target. This means that players do not necessarily need to target the enemy at all, or target it only when necessary.
Because Energy Vampire drains a percentage of the target's maximum Health based on the value when it was cast, it has excellent synergy with Well of Life. If Energy Vampire is cast right before Well of Life ends, its damage pulses will subtract health based on the much larger maximum value used during the latter ability, effectively multiplying the damage of each pulse by a factor of ten after Well of Life ends. This was patched in [Update 18.5].
Link allows Trinity to tank for a short period of time. For 75 energy, Trinity can throw a damage link between herself and an enemy that acts like a buffed version of Rhino's Iron Skin and makes Trinity temporarily close to invincible by allowing only 25% of all damage received to damage the Trinity, while the entire 100% goes to each linked enemy. Any status effects will be nullified on Trinity and transferred to the target. While Trinity's first two abilities end when the target is killed, Link remains active for its full duration and switches from enemy to enemy. If there are no enemies nearby, link will target a teammate (usually a sentinel or hostage), and in case nothing is in range, you will be able to activate it but the damage won't be deflected. Against bosses, this can be extremely valuable; Energy Vampire can return power more than the cost of Link and Energy Vampire combined. Link's range is also very long.
[Update 9.8] Link was changed to target more enemies simultaneously and the range was increased. However, the invulnerability was switched to damage reduction. Link can no longer allow trinity to run into extremely high level enemies with invincibility, or allow her to use Ogris at point blank range for extra damage. On the other hand, she still has blessing for invulnerability and can clear enemies faster.
Blessing is Trinity's final ability. It restores the health and shields of all allied Tenno while granting brief damage reduction at the cost of 100 energy. However, Blessing has a short channeling time of approximately 1.5 seconds after which the healing is applied to Trinity and spreads out in the shape of an expanding dome.
While player items, like Team Heal, do a great job at healing a limited amount of team health, Blessing is not limited by the healing limits of items. A Blessing can replenish all life instantly while at the same time regenerating shields and applying damage reduction across all friendly units. Blessing is extremely useful to heal allies and to avoid damage.
Blessing has infinite range. Teammates can receive a heal and buff anywhere on the map.
Update 9.8 Blessing was buffed to allow instant casts. Although she still needs to do the animation, Trinity is invincible for the duration. This change makes Blessing more helpful as a panic button for when you see teammates with low health.
Update 13.3 Blessing no longer gives invincibility, and instead gives a damage reduction based on the greatest percentage of health healed (if you healed your squad 30%, 50% and 90%, the party would receive a damage reduction of 90%).
Update 14.6 Blessing was changed to restore 40% / 50% / 75% / 80% shields and health of every player and their sentinels, damage reduction system stayed the same as previously. 14.6.1 removed the cooldown period that was previously the duration of the damage reduction.