Aru (New Year) is a returning limited student, sharing banners with 4 other students in the rerun of the event New Years' Rhapsody No. 68. If you don't have her yet, this article will outline whether she is worth pulling for or not.
Aru (New Year) is a Limited 3* Piercing student whose main specialty is clearing tons of mob-class enemies in both regular content like events and story stages and endgame content like Chesed Total Assault.
Unlike other AoE students, Aru (New Year)'s EX skill isn't a true AoE where it hits all enemies in range of her EX skill. Instead, it ricochets and deals damage to nearby enemies. This means that as a mob-clearer, her range is more controlled, since she can hit enemies all over the map as long as they are close enough to each other. Unfortunately, this also means that she can't hit as many enemies as other AoE students, or even deal as much damage due to her EX only having a strict amount of times it can hit.
She does have an interesting niche of being able to better your cost regen whenever her EX skill is used, so you will want to level up her Sub Skill to take advantage of this, as her Sub Skill increases the amount of Evil Deed stacks she can hold. The more Evil Deed stacks she has, the better her cost regen.
A more in-depth review of Aru (New Year) can be found here.
Unless you're a big fan of Aru herself, especially with this specific alt. being limited, pulling on her banner is not recommended.
To start with, There are many AoE students that deal Piercing damage that are just as good if not better options, one of which you can already get for free (Nonomi). Another option is Momoi, who is a 2* and will likely spook your rolls when you're trying to get another student. You can easily get her elephs in Total Assault as well, making it easier to raise her. Iori is another strong student whose EX is similar to NY Aru, as it hits multiple times. Cherino is a permanent student who can easily get target every enemy in a giant circle and can boost cost recovery with no strings attached.
Basically, there are enough alternatives to the role NY Aru plays in battle that if you're considering rolling in one of the banners that she can be found in, you're better off going for NY Mutsuki or NY Kayoko.
Aru (New Year) is another waifu pull who is decent enough to work, so she can still function in Total Assault like Chesed. If you don't need any more Piercing AoE students, then you can safely skip.