Naga

A top-tier Burst 2 supporter who covers ATK support, core damage increase, cover healing, and survival healing all at once.
She has extra ATK and core damage increase lines that unlock only while she has a shield, so pairing her with a tank Nikke that applies shields greatly raises team damage.
In Arena she fills Burst Gauge slowly, making it hard to cycle Full Burst often in the same time frame and hard to gain an edge in short fights.

Each time Shotgun normal attacks quickly accumulate, Support of Friendship2 briefly grants core damage increase to the 2 allies with the highest ATK and heals the 2 allies with the lowest HP ratio based on the caster's final max HP. For longer accumulation, Guardian of Friendship1 heals all allies' cover HP, and when she has a shield, the same activation also grants all allies core damage increase. Her Burst Skill As Long As We're With FriendsB gives herself a Pierce bonus and grants all allies Increased ATK, and when she has a shield, the same Increased ATK line applies once more.

If aiming for the shield condition, place a Nikke that shields Naga in the same squad.
Close the distance so Shotgun pellets do not spread.
Keep her on auto and only aim manually during core phases.

Bastion Cube builds hit count to shorten the activation interval of Support of Friendship2 and Guardian of Friendship1. Resilience Cube fills reload gaps, and Quantum Cube helps refill Burst Gauge. If Harmony Cube levels differ a lot, use the higher-level cube before following the ranking.

Secure Hit Rate so the core damage increase line triggers more often in practice. Put Max Ammo in leftover lines to reduce Shotgun reload gaps.

As Long As We're With FriendsB grants allies Increased ATK proportional to the caster's ATK, so raise it first. Guardian of Friendship1 handles both all-ally cover healing and the shield-conditional core damage increase. Support of Friendship2 alternates core damage increase for attack members and low-HP healing.

She is often fielded in story and bosses as a supporter who also heals cover and provides survival healing. She is worth using in Missilis Tower when conditions fit. In long special enemy fights, she is also considered when healing and core damage support are both needed.

One Burst grants her a Pierce bonus and allies Increased ATK, and if she has a shield, the same line applies twice.
Support of Friendship2 and Guardian of Friendship1 run on accumulated normal attacks, covering both cover and HP at once.

Against enemies without a core, the core damage increase line contributes less.
If her own shield breaks, the extra ATK lines from Guardian of Friendship1 and As Long As We're With FriendsB drop off too.
As a Shotgun, she fills Burst Gauge slowly, which hurts in short-battle modes.