To note: These are first impressions and based on guesswork since we don’t know the exact details.
[Horn] will only be the second defender [Fortress], and the first 6 stars. Like Ashlock, his kit is entirely damage-oriented, which supports their use as a damage-oriented melee sniper-type operator. As a melee unit, their attack range and splash damage offer a unique tactical option.
His first talent will be a welcome sight for Defender-knights players, but other than that it probably won’t have a very noticeable effect, although it will certainly synergize with his own use. Talent 2 is still unknown at the time of this writing.
His S1 stands out for its ability to remove enemy invisibility/stealth, but suffers from the problem that the attack must actually occur for the invisibility to be removed. Since invisible enemies cannot be targeted themselves, this means she must attack a nearby non-invisible enemy to reveal them. If none are present, it will not be able to cloak itself, which limits its applicability.
S2 will be the best skill to use if you want her to block, as it allows her to retain her splash damage when blocking. In normal cases, this archetype will revert to single-target melee damage when blocked, greatly reducing their potential DPS and impact. It also adds an Arts component to damage in Overdrive mode, but mixed-type damage isn’t a very effective option in most cases due to double-mitigation issues. Disabling releases all remaining ammo, which seems like a good burst option and a good way to not waste ammo, and the ammo mechanic itself makes it easy to max out skill effectiveness, even if it requires more manual effort.
S3 is a direct burst damage option, with overdrive increasing the damage even further.
As the first 6-Star Operator to use the Overdrive mechanic, it will be interesting to see how well it works in practice. The downside for her is the loss of HP, which means she will likely need to be covered by some healer of some sort.
As for the meta impact, exact numbers are needed to know how well she will perform as a DPS operator, and her talent 2 is an unknown. However, the meta is generally not very supportive of hybrid roles, so as a defensive/offensive hybrid archetype, she has obstacles in her path from the start.