Steel Cushion

Her signature and the most used option: after her Potential Unlock all her attacks count as back attacks, so the full effect is always active. Its Base ATK is lower than limited engines, but the large DMG bonus keeps it on top. As a Standard engine, its effects double at S5, where it pulls far ahead.

A strong permanent option and the most used one. Its CRIT Rate and Physical DMG suit Corin well. If you fully exploit its back-attack bonus it can even beat Heartstring Nocturne, but most players won't keep that up consistently.

A good Standard Banner pick: his Totalizes can always land from behind for the full +25% DMG. His EX Special Attack repositions him behind enemies; cancel it right after the dash into his Ultimate so the Ultimate Totalize also hits from behind without spending Energy.

Although it's an Attack W-Engine, it's a standard S-Rank with a CRIT Rate main stat, which alone puts it above most A-Rank options.

Nekomata's signature, with a CRIT Rate main stat, Physical DMG and a back-attack DMG bonus. Billy's mobility helps keep it up, but many of his shots push him away, so full uptime isn't realistic.

It has a good CRIT Rate main stat, but the 25% DMG bonus for hitting from behind is very hard to use given how Ellen dashes into her attacks, so in practice it's little more than a CRIT Rate stat-stick.

An S-Rank W-Engine with CRIT Rate, which already makes it decent. Its back-attack DMG bonus suits Evelyn since her EX Special can turn Stunned enemies around, but it demands planning and skill; experienced players can get more out of it than average.

An alternative: Physical DMG bonus (which also applies to Honed Edge DMG) plus extra DMG when attacking enemies from behind.