Hawk Industries Type-97 – ★★★★ Shotgun
CAPTAIN CHINNNAAAAA! Surprisingly good for a free unit. Has the lowest armor among the shotguns (tied with Super Shorty), but she will get you started considering how easy it is to get her. Aura optimized for 1SG/1HG/3MG setups, and works well with Mk23 and M2HB (which are both given for free to new players too). Has a very tiny 3-shot magazine compared to the usual 4 most SGs get which limits her firepower somewhat, but her skill offsets this a bit (not that shotguns get to use their skills besides SAT8). A flawed unit and not a first pick for many lineups, but most SGs are interchangeable as far as front-row tanks go anyway, even if she's not as good at it as the other ones you'll get eventually. Her main merits are a decent aura shape and being free.
Has pretty decent tiles, but since tile buffs are diminishing, she is slightly less good than you would likely think. If you are using Mk23, she only gives a 8.8% damage increase (148/136). Still better than nothing, but may not bring you to another breakpoint for most enemies (if an enemy needs 4 attacks to kill, the 8.8% is probably not going to bring you to 3 hits).
But if you don't have any other special SG, don't overthink it. She does have the weakness of having only 20 Armor, though, although it doesn't matter often as well (It only matters in specific cases where you would take 1 damage with 22 or 23 armor, but up to 3 or 4 damage with 20 armor, which then you have have less effective HP.)