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The latest patch comes with a new item in the Golden Thread shop called "Costume Selective Enhancement" that allows you to upgrade your costume. It is available to buy once a month.
This is a far more effective way of getting dupes of costumes than pulling on the select banner, which has really low rates of getting the costume you want.
This guide sets out some recommended units to spend on.
Adventurer of the Unknown Diana is the best supporter in the game. Having her at +5 is necessary to complete a lot of end-game content. Although she only works for on-element attackers, this is not as niche as you might think: for end-game content, you are almost always using on-element attackers to maximize damage.
She is therefore recommended as universally useful unit to get to +5.
Homunculus Lathel and B-Rank Helena
For H.Lathel, getting him to +3 is recommended for physical teams, especially for use in PVP. The power boosts at +4 and +5 are relatively small.
For B.Helena, there are big power boosts at +4 and +5.
Pool Party Angelica
Pool Party Angelica is one of the highest utility units in the game, thanks to her massive damage against strong opponents. At +5, allowing for chain damage, Angelica's damage is (0.25 + 0.25 x 1.1 + 0.25 x 1.2) x Max HP of the target. Simplified, that means 82.5% x Max HP of the target before crit.
However, the "Max HP" part of the equation is capped at 20,000 HP. This means that without crit, Angelica's max damage against an enemy with 20,000HP or more is 82.5% x 20,000 = 16,500 magic damage.
Unlike most units that have a base crit dmg of 50%, Angelica's base crit dmg is 100%. She also has a unique weapon which increases her crit damage by 90% at UR level, so it's easier to reach high crit dmg. For example at 400% crit dmg with crit on all 3 hits against an enemy with 20,000HP or more, Angelica would deal 82,500 magic damage.
With both excellent damage and excellent AOE, she is a universally useful costume that will help you clear tough PVE content. She is also one of the hardest hitting units for Last Night mode. Her main downside is that her damage potential is capped because she does not benefit from M.ATK supporters.
White Cat Rou (for PVP)
If you have White Cat Rou at +1 or +2, getting her to +3 is a worthwhile investment for PVP play. At +3, White Cat Rou costs only 1 SP, making her easy to slot into any team. She has a reasonable chance at dealing lethal damage with her knockback, and then on your opponent's turn she is a tank that is very hard to kill with her evasion ability.
Your favorite 5-star costume
Finally, if there is a 5-star costume that you use often, by all means upgrade it.
Note that most costumes have their SP cost reduced at +1 or +3. The SP cost reduction makes a big difference (especially in PVP), so also consider which units you need to have the SP cost reduction the most.