How good is guard?

If ran with the Quick Riposte seal, then it’s good on enemy phase. On player phase, it holds virtually no value at all.

If you plan to use her in higher levels of arena, guard is invaluable to take on bold fighter armors, brave lyn, reinhardt, ayra, etc. Especially since you're warding breath instead of steady breath, you're really gonna want guard to take on those units.

Higher Levels of Arena contain a bunch of scary units with builds that can get off their specials faster.

One example is Bold Fighter Zelgius with QR Seal and Steady Breath.

Guard is useful alongside QR Seal to block a lot of those builds.
On Nowi, that can be potentially useful against even some units like Ayra that commonly runs Slaying Edge from procing their specials.

I love Guard.

Guard is one of those skills that is really good, but only on a select few units. Lukas, for example, can make really good use of Guard to avoid being killed by Special Triggers like Black Knight's Black Luna, taking a green unit's special trigger to the face (since he can tank green units pretty well.

Nowi, however, may make better use of a different B skill. Quick Riposte is one of her best B skills, especially if the Quick Riposte seal is being used on another unit. Vantage is also a good skill to use on her. So it really depends on what build you're trying to use.

Allow me to shed some light on the subject! Short essay incoming! ^_^

I've been using Guard 3 Nowi for quite a while now; around a month. I have a +Def -Res Nowi, and run her with Lightning Breath+ (Def), Rally Def/Res, Aether, Steady Breath, Guard 3, Infantry Pulse 3, and Quick Riposte 3 seal. There's three things that you need to know when it comes to Guard.

First of all, it's an outstanding safeguard. Opponents who rely on specials for damage, even quite a few Axe units, are suddenly neutered. Nowi can go toe-to-toe with a vast amount of the units in the game with Guard. She becomes a fairly reliable check to most Lance units, beats every non-Falchion sword unit (although mine's still beaten some Falchion users in enemy phase), and she crushes most archers when beefed up on the physical side. Notably Brave Lyn, who often runs Quickened Pulse to try and get a cheeky double; with Guard, you can lure her in, take minimal damage, and then retaliate. Essentially, Guard 3 vastly expands your amount of heroes you counter, and makes sure the ones you do counter are countered 100% of the time. The other two notable sure-counters are Arya and Zelgius, who can both be a hassle for certain other Blue units to handle.

The second thing you need to know is that it's a pretty needy ability. It only really shines when partnered with the Quick Riposte seal. On Nowi, anyways. This is because the unit needs an incredibly strong Enemy phase in order to really get the ability's value. I've tried running Guard Nowi without it, when I swap the seal to someone else briefly, but you just lose out on a stupidly high amount of damage. Like, your damage is halved. Which makes sense, since you're hitting the enemy half as many times.

Finally, you have to watch out for the Hp threshold if you're not running a healing special like Aether. I used to run my Nowi with Bonfire, and it was still amazing, but you lose a lot of value once Guard is pushed out of range. And it's not that difficult to do if you make a mistake, since 80% isn't very forgiving. However, if you do have Aether, you don't ever really have to worry. Every since I gave my Nowi Aether, she's had no problems staying in Guard's range.

Essentially, Guard 3 Nowi with Quick Riposte seal is an absolutely incredible set, and I highly recommend it. However, keep in mind that this means your QR seal is occupied. I'm going to give my Nowi QR in her B-Slot soon, because I need to put the seal on a different hero. So if some other hero has QR reserved, then don't go for Guard. But if it's open, then by all means - unleash the beast!

NOTE: Just because Guard Nowi *can* handle a stupidly high amount of units doesn't mean I advise throwing her around wildly. I know it sounds obvious, but you should still play cautiously with her like you would with any unit. Think of it as a safeguard.