Unlike many other games, Hoyoverse has made sure that the four-star characters in Honkai: Star Rail isn't something you can look down upon. Luka was first introduced to the game in the second half of the Version 1.2 update, and he follows the path of Nihility, known for the application of debuffs on the enemy.
The best part about the four stars is that they're added to the Standard pool of characters after their first banner, which means you can always get them. While Luka can be decent by himself, he's much better when used in a team with specific characters.
7 Kafka
Kafka has been one of the most anticipated Nihility characters ever since the start of the game, and she goes quite well with Luka. The special part about her skill set is that she can instantly trigger all DoT effects on a certain character whenever she hits them with her Skill or Ultimate.
Luka can apply the Bleed DoT on one enemy through his Skill or if he breaks their Toughness Bar. Your Damage Over Time scales with your ATK while the Break Effect becomes better with your character level, which means that you'll have to upgrade both of these characters to level 80 if you want to get maximum value.
6 Asta
Asta is likely one of the most underrated characters in Honkai: Star Rail since she can provide a ton of ATK and SPD to the entire party. If you're fighting a horde of enemies, she can easily stay at five Charging stacks all the time, giving you one of the highest ATK boosts in the game.
Moreover, the SPD provided by her Ultimate lets Luka get his turns faster, which in turn helps him use the instant DoT application from the enhanced basic attack. His Skill's DoT already lasts for up to three enemy turns, but it feels much better when Luka gets two turns before an enemy gets one.
5 Pela
Talking about underrated characters, Pela is an Ice Nihility character who can massively reduce an enemy's DEF when her Ultimate is used. This debuff is applied to all the enemies on the field, and it can stack with Luka's Ultimate, which increases the damage dealt to one enemy for the next couple of turns.
By himself, Luka can only apply up to two debuffs to an enemy. If you want to build him as a DPS character and use the Good Night And Sleep Well Light Cone on him, Pela's extra debuff helps you get the final stack. You can also use these two characters as support for other carries.
4 Seele
Talking about being a support for a main DPS, Seele was the first limited banner character in Honkai: Star Rail. Her skill set allows her to deal multi-target damage even though she's a dedicated single-target DPS. You'd always want to use supports that increase her damage on the enemies, and Luka can do just that.
Of course, you'll need as much Energy Regeneration Rate as possible since the increased damage from his Ultimate only lasts for two turns. Apply this effect on a boss enemy and you can easily defeat them using the extra turns you can get with Seele due to her Bonus Abilities.
3 Gepard
Honkai: Star Rail features a turn-based combat system where all your characters are susceptible to getting attacked by enemies at any point. As the game gets older, the number of enemies that can one-shot your characters will also increase. This is especially true for a squishy character like Luka without any self-sustaining ability.
If you want to benefit from his DoT and other damage types, you'll have to keep him alive for as long as possible. Apart from the shield, Gepard also has a higher chance of getting attacked by the enemy. Moreover, he can also force an enemy to skip their turn with his Skill's Freeze.
2 Sushang
Sushang is a four-star Hunt character who has Physical as a combat type. Much like Luka, she's also great at breaking an enemy's Toughness bar. If you don't want to build Luka completely on Break Effect, you can also trigger this Break with Sushang since it still applies the Bleed DoT on the enemy.
Along with the DoT from Luka's Skill, this gives you the option to apply this effect on multiple enemies, which increases your options for the enhanced basic attack. Moreover, Sushang's first Eidolon lets her recover a Skill Point when her Skill is used on an enemy whose Toughness is broken.
1 Silver Wolf
The Memory of Chaos is the hardest content in Honkai: Star Rail and it can be quite discouraging when the enemies don't have a specific Weakness. This is especially true for Luka since he makes use of the Physical Weakness to apply more DoT instances on an enemy.
Silver Wolf can help you in these situations since her Skill can apply one of the combat types in your party as the Weakness on the target enemy. Moreover, her Ultimate can also further reduce this enemy's DEF, which increases the overall Physical DMG dealt by Luka.