
Free Agent - EU
Rei, isn't good as a solo Support
Rei, is good in double support meta, because she deals a good amount of damage reliably, she's stronger than damage Jenos for sure, but her ult is more of an I win button than an I start the round by erasing 1 or 2 of you because I'm so good at Jenos.
Just remember: in low elo, anything can work, in casuals, anything can work, this guide is for those who for some reason play ranked, and want to know why they're doing bad on their main, Rei.
Rei has the same winrate% of 55% on all of her talents in season 4 at dia +, however, she has less than 5k games all together in dia+.
Let's explain what each talent actually does so that we'd know why.

Extension
Increase the range of Chain Heal bounces by 40%.
Level 1
Extension: Increase the time until your cooldown actually starts by up to 2 seconds because your bouncy heals decided to go to the farthest ally who has full health.
In lower elo, Extension has the lowest winrate at 52%, with the highest pick rate at 20,964 matches.

Restraint
Spirit Link
While Envelop is active and you are Linked to an enemy, prevent them from Healing.
Level 1
If you use your survivability (Envelop) while linked to the enemy tank, they can't be healed, this is what you use to bully Azaan in casuals, but it requires a stack.
This is what you would use if you don't want to play Focus, as it has no drawbacks and Rei isn't meant to solo support anyways so you don't need Extension, however, don't bother using this talent unless your enemy has no shield (or you just broke it), otherwise, you wasted a long cooldown for nothing.

Focus
Spirit Link
Spirit Link's Cooldown is increased to 7s and it now only lasts for 4s, but its base effects are tripled.
Level 1
As a main support: you now have a 2nd source of healing that can grant up to 250/s for 4s (exactly 1K healing over 4s which will obviously be Cauterized mostly), which is actually on a 1.9s reuse cooldown since the cooldown of Spirit link starts as soon as you link to someone, so after 4s of it being used to link to someone, you wait 1.9s (under Chronos 3) to get the link again, however, your ult depends on you linking to someone, so saving your link for when it's needed most might be the play here.
As a damage: deal damage and slow enemies on a CC that can only have its effects reduced by Resilience, but not its duration, Spirit Link can kill damage immune enemies for some reason, most often seen on Vora while she's ulting you, wherein you'll kill her before she executes you, but, you will die to a dead Vora ult, WOW!.
Anyways, as an off support, Rei is expected to have tons of damage and tons of kills/eliminations, Rei into a flanker is so cancerous and annoying, but, you're still doing it at the expense of having 1 less tank, good luck with that!.
Rei has the weirdest defensive ability in the game, Envelop makes blasters and slow firing champs rage, but, remember, it's on a long cooldown, all of the above can deal with Rei, eventually.
you're still just a support, don't forget that, use your 2 I win buttons back to back to really make them rage.
Console champs make Rei only benefit from the speed gained by Envelop, since there's literally no reduction to these champs damage, not that anyone would specifically pick one of these to counter you, all it takes to counter Rei is to shove a tank in her face or just to bait her cooldowns.
So many Rei players that I see, don't even read, sad, what's worse is the cringe guides available all over that have too much cringe info in them, so here's why that info is cringe:

Arcane Focus
Spirit Link
Heal for 20 every 1s while Spirit Linked to an ally.
Level 1
Rei heals herself with her ability, why would you need an extra source of healing?

Threat and Subject
Armor
Increase your Movement Speed by 8% for 5s after earning a Killing Blow or Elimination.
Level 1
forget everything else, it's 2s of movement speed after an elimination, really? useless, not even if you want to snowball.

Aico Forces
Chain Heal
Reduce the damage you take by 1% for 2s for each ally hit with Chain Heal. This effect stacks.
Level 1
You don't need this card, it literally is a waste of points, not even as a filler, if you want DR, buy Haven later in the match.

Arcane Maelstrom
Envelop
Using Envelop reduces your linked friendly target's Cooldowns by 4%.
Level 1
Let's use a long cooldown to reduce my linked ally cooldowns by virtually nothing! YAY! -_-

Deft Magics
Spirit Link
Linking to a new target reduces all your Cooldowns by 0.1s.
Level 1
My team is literally stacking and close enough for me to spam linking instead of spamming damage, because I want to fight my team not my enemies.... yeah right.

Enveloping Magic
Envelop
Increase the maximum amount of damage mitigated by Envelop by 7%.
Level 1
let's assume you're using this at 2 points ok?
first, your base Envelop damage limit is 275/instance
14% of 275 = 38.5
you now take 236.5 max damage per instance
now YOU tell me if it's worth it.
Hint: it's not worth it.

Leipori Prayer
Chain Heal
Grant 50% Reload Speed for 0.5s to each player to which Chain Heal bounces.
Level 1
Legend says that once upon a time during a stomp, a Raum reloaded so quickly he didn't even know what happened, but it only happened once.
this card requires 2 or more points for it to have any value, so don't use it as a filler, 0.5s is literally not long enough unless whomever is getting healed notices and decides to reload right then and there.
Here's the usual stuff you run, why? logic, simple stuff.

Spring to Action
Spirit Link
Increase the Healing Spirit Links provides every 1s to your linked friendly target by 100 Health.
Level 5

Magic in the Blood
Weapon
Reduce the Cooldown of your next Chain Heal by 0.5s after hitting an enemy with Sigil. This effect stacks.
Level 5

Refreshing Break
Chain Heal
Grant 0.51% Ultimate Charge to each player to which Chain Heal bounces.
Level 3

Rest and Relaxation
Weapon
Increase your maximum Ammo by 2.
Level 1

Future Generations
Chain Heal
After taking -125 damage, your next use of Chain Heal does not cause it to go on Cooldown.
Level 1
Future Generations is a must at 1 point in all of your loadouts.
you can move 1-2 points from Spring to Action to Refreshing Break if you want to have more ults, which is usually the better option here.
if using Focus, this gives you 250/s for 4s on your linked ally.

Refreshing Break
Chain Heal
Grant 0.8500000000000001% Ultimate Charge to each player to which Chain Heal bounces.
Level 5

Magic in the Blood
Weapon
Reduce the Cooldown of your next Chain Heal by 0.4s after hitting an enemy with Sigil. This effect stacks.
Level 4

Midnight Stroll
Spirit Link
Reduce the damage done by any enemy you are linked to with Spirit Link by 16%.
Level 4

Future Generations
Chain Heal
After taking -125 damage, your next use of Chain Heal does not cause it to go on Cooldown.
Level 1

Rest and Relaxation
Weapon
Increase your maximum Ammo by 2.
Level 1
Link to the enemy flanker to take 16% less damage from them.
You're a noob so you won't be hitting that many shots on anyone anyways, so no lost value from less points on Magic in the Blood.

Midnight Stroll
Spirit Link
Reduce the damage done by any enemy you are linked to with Spirit Link by 20%.
Level 5

Rest and Relaxation
Weapon
Increase your maximum Ammo by 4.
Level 2

Arcane Manipulation
Envelop
Reduce the Cooldown of Envelop by 1.5s.
Level 5

Mystic Mastery
Armor
Increase your maximum Health by 100.
Level 2

Future Generations
Chain Heal
After taking -125 damage, your next use of Chain Heal does not cause it to go on Cooldown.
Level 1
This loadout is used by a certain player terrorizing dia+ by going flank Rei.
You're not skilled enough to make good use of this, trust me.