If you are starting out in Chaos Zero Nightmare, building up some of these characters would allow you to beat the content while saving a lot of resources. However, since the game is not that hard, you can just get some core units and level the other units with aesthetics you like.

Mika

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Mika is an AP Support (Controller), and her main strength is being the only character in the current version who can reliably restore Action Points (AP). Her AP regeneration skill really helps the whole team's skill rotation cycle, letting main DPS characters use high-cost skills more often. This greatly improves the team's overall efficiency and damage potential, making her essential for any team that relies on skill combos.

Khalipe

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Khalipe is a Vanguard, but she also packs a lot of damage. Her core card, "Vulture Launch," has the Celestial tag, which triggers automatically when you play another 2-cost or higher card, dealing AoE damage and creating a shield, then returning to hand through the Retrieve effect. This setup lets her keep dishing out damage and protection without using extra AP, clearing enemy groups effectively and restoring AP for the team via Break damage. During testing, she stood out as the most balanced frontline character.

Mei Lin

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Mei Lin is a fire-element core focused on Single-Target DPS (Striker). She has the unique ability to apply Elemental Weakness. Her starting card gives her this effect permanently, making all following fire-element attacks on the target deal an extra 25% skill damage and more Break damage. This helps fix team elemental gaps, boosting the team's Break efficiency a ton. The AP gained after breaking an enemy's Break gauge also smooths out skill rotations.

Orlea

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Orlea is a versatile Controller (Healer/Summoner). She can summon and evolve three types of creatures: offensive, supportive, and defensive. These summons begin at 1-star or 2-star and can fuse into a stronger higher-star version when two of the same star level are in hand. Fully evolved 3-star defensive summons offer huge healing or shields, and thanks to the Retain effect, they provide ongoing benefits without using AP.

Kayron

Kayron Chaos Zero Nightmare

Kayron is a Void-element AoE DPS (Psionic) and a key unit for dark teams. He boosts his burst skills by creating and using "Futility" cards. When these cards are Exhausted, his main damage skills get lower AP costs or higher damage. In testing, he showed some of the best AoE damage, but he needs a specific team setup to exhaust his Futility cards fast.

Renoa

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Renoa is a Void-element specialist in Single-Target Burst damage (Hunter). She creates special "Bullet" cards that aren't played directly, but they trigger extra attacks automatically when she uses her other skills. The more bullets she has, the more extra attacks happen. She's one of the top single-target damage dealers in the game, especially for bosses with high HP, but her burst cycle takes longer, needing enough bullets built up to hit full power.

Hugo

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Hugo is an Order-element Combo-focused DPS (Hunter), a flexible wind attacker. His core skill gives him stacks of "Hunt Commence" when allies use non-attack cards. When any character deals single-target damage, he uses one stack to follow up with an attack. Hugo's damage doesn't depend on AP; putting him as a sub-DPS in the team delivers solid damage with great resource use.

Veronica

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Veronica is a Passion-element Auto DPS (Ranger). After playing her upgrade card at the battle's start, she automatically creates "Ballista" cards each turn. These cards trigger at the turn's end, hitting random enemies and using zero AP. Once going, she delivers steady free damage every turn, and her other skills help the team draw through the deck, making her a great sub-DPS that doesn't mess with the main carry's attacks.

Rin

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Rin is a Void-element extreme damage dealer (Striker), a selfish hyper-carry. She ramps up her damage big time by entering the "Dark Cloud" stance, but it ends if an ally uses an attack card. In perfect setups, Rin has some of the highest single-target burst, but her team needs to be very specific, with teammates who avoid attack cards, like pure supports, to keep her stance active.