Rei is known as the ult charge support, but I think she holds a much stronger niche as the damage reduction support. Midnight Stroll is by far her strongest card in my opinion, and I consider it one of the strongest cards in the game. The ability to just lock on to an enemy with an auto targeted, infinite duration, 20% damage reduction is just absurd. You are the true anti carry. Anyone who dives your backline will suddenly be drastically weaker than what they are used to, and you can reduce the impact of a lot of ultimates. Combined with Aico Forces and your defensive abilities and you can turn anyone on your team into an immovable object.

During combat I prioritise linking to enemies for the damage reduction, but swap to an ally before casting chain heal if i think they need the 30% bonus. When there is no enemy to attach to, I always attach to the ally with the most important ultimate who isn't already at 100% charge. At the start of a round it's important to link to an ally before getting on your mount and then follow them to the battle in such a way that it doesn't break.

Magic in the Blood (lvl 1-3) and Future Generations (lvl 1) are must haves. Future Generations activations aren't lost on death, so if it was up when you died, you'll usually be able to double cast when you get back to the battle. You'll still have to wait for the first one to stop bouncing before casting the second one though. This is one of the reasons not to use Extension, as it often spends more time in the air. Magic in the Blood can be adjusted in level, but I find that lvl 1 is usually enough for this build.

Threat and Subject is such an underrated card. Getting eliminations on a support is super easy and the extra movement speed is extremely useful. Controlling LOS is one of the most important skills on Rei. It's important for controlling chain heal bounces and to avoid breaking spirit link. Making sure to always LOS a teammate is also what enables us to throw hands with pretty much any character in the game, because we need something to bounce chain heals off of. The extra movement speed helps with all of this. It also helps us get to people who need saving a lot quicker and to finish off low targets who are trying to run away. I get extra saves and kills from this card in almost every single game I play using it.

For items I always start Chronos and Nimble. Chronos doesn't really need an explanation and Nimble is important for the LOS control I just talked about. After that I get Resilience if needed. Otherwise I just go straight for Kill to Heal. Kill to Heal is very strong on support because of how easy it is to get eliminations. It also isn't affected by anti heal, so it's really good as a 3rd or 4th item. If I don't need Resilience I tend to gravitate towards Morale Boost or Lethality in the 4th slot, but wrecker is also a nice choice into certain comps. And because healing falls off in the late game, it's nice to have an option that can boost your damage.

Update:

The new item changes don't affect our build path all that much. Chronos and Nimble are still 1st and 2nd. The addition of Armor Plating is really nice as Envelop tends to be a lot more useful against abilities. Arcane Warding is more situational, but great into characters with a lot of DOT or chip damage abilities like Omen, Grohk and Lillith. Sentinel is awesome as it covers the cc that tends to be the most problematic for us while simultaneously acting like a pseudo Kill to Heal since shields are unaffected by Cauterize. Unbound is obviously great into relevant cc. Blood Bath is technically an even better support item than Kill to Heal was, but I find it difficult to justify building early in Rei, which means we'll have to worry about Cauterize. Other supports use it a lot better.

My go to build this patch has been:

Chronos -> Nimble -> Armor Plating -> Master Riding

As stated previously I don't think Focus is Rei's best talent. But it can be very strong as an offensive tool in combination with Midnight Stroll. The extra damage and slow combined with the 20% damage reduction can completely strip almost any target of their agency.

I run this build when:

1. My team has a second support.

2. My team doesn't have a flank / my other support is good at supporting flanks.

3. The enemy team does not have Evie, Moji, Koga, Zhin or Talus.

A second support makes the hit we take to our other supportive abilities at least somewhat justifiable, and the increase to our damage is very helpful in double support.

Rei is really good at pocketing flanks, but Focus hits this really hard because of the added cooldown and lack of team targeting on Spirit Link. If the other support is specifically designed to support flanks, I'll still run this build. But otherwise it's not worth it.

The flanks mentioned are made extra difficult to deal with by running Focus because of their ability to cleanse it and put it on a long cooldown. Seris can also do this, and Atlas messes with it regardless of which team he's on. An enemy atlas can rewind you or himself to cleanse it, and a friendly one will sometimes rewind the enemy you're attached to, accidentally cleansing them.

This deck has a lower focus on movement speed because our slow is stronger and because our ability to perform clutch saves is worse. Nimble is still nice to round out the build at the end, but we max Chronos before touching anything else.

Magic in the Blood lvl 3 helps make up for the loss of defensive utility, because it increases our healing output and helps us apply Aico Forces more often. It also synergises well with our increased focus on dealing damage. Rest and Relaxation also adds to this synergy.

The more I use this build, the more I start to see linking to an ally as a waste of resources. I will of course always try to save people with Envelop and Vivify if the cooldowns line up when I need them. But linking to an ally for the bonus ult charge is a decision I regret almost every time. Keeping it available for an enemy or to save someone is almost always more valuable. There will always be exceptions, and you can't spreadsheet everything, but I recommend being careful linking to allies during downtime.

Don't be afraid to solo Vivify or solo Envelop. Regardless of what build you are playing you can't keep your team alive if you yourself are dead. There are few things worse than duelling a Focus Rei with Midnight Stroll lvl 5. One of them is duelling one that isn't afraid to burn Vivify.