The best Robin build in Honkai: Star Rail is all about building attack for Penacony's pop star and prima donna. Like Luocha, Robin is a support character whose abilities scale with attack, meaning she can both deal a little damage and provide potent buffs to the entire party. Where she really shines is with her Vox Harmonique, Opus Harmonique ultimate, which causes her to enter the Concerto state.

As she belts out a song, every party member has their action advanced forward and receives a damage buff for a bout of musical murder. Robin also follows up their attacks with additional physical damage that scales with her attack as well. Add to this a skill that straight-up buffs partywide damage, and you have the most powerful support character since, well, version 2.0 with Sparkle. All that said, here's my favourite setup for Robin.

The best Robin build

Robin is one of those slightly strange support characters who's also capable of dealing decent damage. Since the attack buff her ultimate provides to allies scales with her own attack, it's relatively simple to just build that stat, increasing the buff's potency as well as how hard Robin can hit. 

During her ultimate's Concerto state, she'll deal additional damage to enemies after allied attacks, also scaling with her own attack. It makes her a straightforward character to build, even if her kit's speed and energy regen does make some of the regular support character light cone and relic picks a bit pointless.

• Light Cone: Flowing Nightglow or Memories of the Past

The best five-star light cone for Robin is her signature Flowing Nightglow. This cone is pretty cracked, granting one stack of Cantillation every time an ally attacks, with each stack buffing the wearer's energy regen by 3%, up to five stacks. When the wearer uses their ultimate, Cantillation is transformed into Cadenza, which increases the wearer's attack by 48% and the team's damage by 24% for one turn. 

If you're looking to save your jade for other characters, Bronya's But the Battle Isn't Over continues to be a solid harmony light cone, buffing energy regen and increasing damage for the next character who acts after Robin uses her partywide damage buff skill. If you already happen to have Ruan Mei's five-star, Past Self in the Mirror, this will also be an effective replacement, since it buffs allied damage when Robin uses her ultimate.

For four-star options, your best bet is Memories of the Past, which grants a little buff to break effect and regenerates four energy when the wearer attacks, since this'll help Robin use her powerful ult even more.

• Relics: Musketeer of Wild Wheat (Two-pieces equipped), Prisoner in Deep Confinement (Two-piece equipped)

The two best relic sets for Robin are Musketeer of Wild Wheat and Prisoner in Deep Confinement, both of which provide a 12% attack boost for two-pieces. For a support character, I'd generally recommend a set like Messenger Traversing Hackerspace for its partywide speed boost when using an ultimate, but the problem here is that Robin's ultimate advances everyone's action forward, making that totally pointless. 

For stats, you'll want an attack bonus on the body and either attack or speed on the feet. I'd generally lean towards attack more than speed for Robin, just because she can't act while she's in her ultimate's Concerto state, making lots of speed a little pointless, and when it finishes her action is immediately advanced forward anyway. 

Robin should get a lot of energy regen from her light cone, Sequential Passage bonus ability, and her Tonal Resonance talent, meaning you can safely invest in attack without having to worry about it too much. You might also want to build some HP for survivability, considering her importance in supporting the rest of the party. 

• Planar Ornaments: Fleet of the Ageless (Two-pieces equipped)

Usually, I'd suggest Broken Keel for its partywide crit damage bonus, but it requires building effect res, and Robin is already immune to crowd control debuffs during her ultimate's Concerto state, limiting the value that stat would provide. You could still totally use it, but I'd recommend Fleet of the Ageless instead for its HP boost and partywide attack bonus, provided you get over 120 speed. It'll increase the potency of her ult buff and make her a tad tankier.

Stat-wise, you should try to get either an attack or physical damage bonus on the sphere, then either energy regen or attack on the rope. Personally, I'd lean towards attack on the rope, just because Robin gets so much energy regen through other sources, but there's no such thing as too many ultimates.

Robin abilities and Eidolons

Here are Robin's abilities, traces, and the Eidolons you can unlock by acquiring extra copies of her—thanks Honey Hunter for the info:

Robin Ascension materials

Here's what you'll need to ascend Robin, not including XP books:

• 15 Dream Collection Components
• 15 Dream Flow Valves
• 15 Dream Making Engines
• 65 IPC Work Permits
• 300,000 credits (approx)

You can get Dream Collection Components—and the higher rarity valves and engines—by defeating Dreamjolt Troupe enemies in Penacony and through the A Startling Night Terror assignment. The best way I've found is doing the Penancy golden calyx, as you can farm up XP books and credits at the same time.

For the IPC Work Permits, these won't be available until version 2.2 launches. You'll have to head for the Stagnant Shadow: Clock Studios Theme Park in Penacony to grab them once it does. As for credits, you can always farm golden calyx to get what you need.