Redeemer

 is a heavy attacker and blocker unit. It has the downside to build Gauss Charges for your opponent.

Strategy
Redeemer is at its best when used to rush down an opponent going for a greedy and defensive strategy that prevents them from pressuring you for a while, such as Savior or Zemora Voidbringer. In these situations, the incoming Gauss Charges will hurt a lot less since the opponent has little to no attack to back them up. If you manage to even partially absorb the Charges, even better.

is also a good buy if you can soak the Gauss Charges with an efficient defender like Plexo Cell or Aegis, or late in the game when the Gauss Charges will arrive too late to make a difference. (Note though, by that point, buying Attackers is not that good either. Attacking for 2 turns being worth about 9, it may be net value still as the enemy must destroy it.)

Redeemer is a risky purchase if the opponent has access to units such as Grenade Mech or Nivo Charge, as you will already be hard-pressed for defense on the Gauss Charge turn, making an additional burst of sudden pressure even more devastating. Opponent may also exploit the charges effect by means of attack consuming units, most notably Arka Sodara, since it can go tech more easily to perform such counter. The user can use the effect fo its own advantage instead though, by setting to get Arka right before the enemy charges hit, ensuring no counter denial.

Can combo well with non-prompt sources of defense such as Shredder, as these allow you to efficiently soak the Gauss Charges starting the turn before, spreading out the impact a bit (which is helpful since you can only buy so much prompt defense in one turn before you have to turn to inefficient sources like holding back Drones).

It can be considered a 10 2x Attacker with an extra effect: Get an extra attacker for needing 6 defense in 2 turns. Often this may be slightly negative, but the resource cost means it can add pressure efficiently. Having only Wall as efficient soak, the first 2 extra attack can be taken as pay for 3 of the Charges, then getting a Wall for the other 3 ties for having the extra attacker from then.

Openings
The following rush is common for Player 2 in Redeemer sets:
 * DD
 * 1) DDC
 * 2) DDB
 * 3) D + Redeemer

Rushing out a Redeemer like this puts 3 constant damage out fairly early, which can be quite annoying for an opponent who does not have a Blastforge yet and thus cannot get good absorb.

Player 1 can get it on Turn 4 as well.
 * DD
 * DD
 * 1) DCB
 * 2) Redeemer
 * 3) Redeemer +E
 * 4) WFDE

Player 1 high Econ with Infusion Grid (2). Player 2 has a much easier position for this, since it can buy exactly to open with Redeemer but then exploit natural Infusion Grid and delay their defense 1 turn.
 * DD
 * 1) DDE
 * 2) DDD
 * 3) DDDC
 * 4) DDBB
 * 5) Redeemer +WD
 * 6) Redeemer +DE2 (x2 Redeemer if opponent lacked pressure)

Player 2 high Econ with Doomed Wall (1). A strong head-on rush.
 * DD
 * 1) DDE
 * 2) DDDC
 * 3) DDDB
 * 4) DDDB (3B left over)
 * 5) 2x Redeemer
 * 6) 2x Redeemer

Player 1 may use Doomed Drone (1) for a rush build. They also Soak the charges.
 * DD
 * 1) C11
 * 2) BD1
 * 3) Redeemer +1
 * 4) Redeemer +1

Most aggressive Rush for Player 2 with Doomed Drone (1). This has the effect of locking the opponent from getting attackers back. Galvani Drone also works, though a lot weaker.
 * D1
 * 1) CB1
 * 2) Redeemer.
 * 3) Redeemer.
 * 4) W...

Rush of Player 2 with Borehole Patroller (1). Urban Sentry works much like it, but does not prevent opponent's Sentry. The variant (Turn 3 and 4 swap) is not recommended, unless Urban Sentry is the extra unit and opponent is without preparation.
 * DD
 * CB
 * D1
 * 1) Redeemer
 * 2) Redeemer
 * 3) W... (May absorb on 1)

Player 1 may rush with a Shredder (1) if Doomed Drone (0) is present. This blocks opponent's Redeemer.
 * D0
 * 1) B00
 * C1
 * 1) Redeemer
 * 2) Redeemer
 * 3) W...

Player 2 has an exact Thorium Dynamo (1) and Doomed Drone (0) opening.
 * 01
 * B
 * 1) Redeemer
 * 2) Redeemer
 * 3) W...

Player 2 with Auric Impulse (1).
 * D1
 * 1) CB1
 * 2) Redeemer +1
 * 3) Redeemer (E vs expected Attack)
 * 4) WFD...

Player 1 may rush with Centurion if Auric Impulse (0) is present. This absorbs the Gauss Charges.
 * DD
 * 1) DDE
 * 2) DDD
 * 3) DDDB
 * 4) DCS0
 * 5) Redeemer +000
 * 6) Redeemer +CBA00
 * 7) Centurion +TD

History
has been through a large number of significant nerfs, buffs, and reworkings because many versions of it have been extremely player-advantaged with very sharp openings. This video goes through the history of and the reasoning behind the changes.

Change log

 * April 6th, 2018
 * Cost changed from to.
 * May 27th, 2016
 * Cost decreased from to.
 * Now gives opponent 6 Gauss Charges with instead of 5.
 * October 15th, 2015
 * re-added to game.
 * Cost increased from to.
 * Now gives opponent 5 Gauss Charges with.
 * October 13th, 2015
 * temporarily removed from the game.
 * September 28th, 2015
 * re-added to game.
 * Cost increased from to.
 * The 6 Gauss Charges constructed when purchasing a are now distributed as follows: 3 with  and 3 with.
 * September 22nd, 2015
 * temporarily removed from the game.
 * September 18th, 2015
 * Cost decreased from to.
 * Instead of constructing 4 Gauss Charges for the opponent, purchasing a now constructs 6 Gauss Charges: 2 with, 2 with , and 2 with.
 * April 7th, 2015
 * Cost changed from to.
 * February 19th, 2015
 * Added to the game.