General overview

No operator has had popular opinion about them change nearly as drastically as Passenger. He was considered one of the worst 6 stars on release, but balance updates caused some people to change their minds about him. Add in his modules, and he's now considered one of the best casters in the game. That being said, he's very good even without modules, provided you temper your expectations about what he can do. His crowd control isn't quite as good as dedicated slowers, his DPS isn't quite as good as some other casters, but he's substantially better at handling large crowds of enemies than a slower or caster would be on their own.

E2? Basically mandatory, because skill 3 is very good and access to his modules does a lot for him. His chain length also increases to 4 from 3 at E1.

Talents

Talent 1 (Mechanism Analysis) gives Passenger a temporary damage increase against enemies with over 80% HP. This is a nice buff that helps him weaken enemies as he slows them from afar. Talent 2 (Lone Soldier) gives him an ATK buff if there are no enemies in the surrounding 4 tiles. This buff isn't terribly hard to take advantage of, and you'll often get it for free by virtue of map design.

Skills

S2 (Focus Command) gives Passenger a range extension, slight ATK increase, slight attack interval decrease, and increases his maximum number of chain jumps to 5. This skill is often overlooked, but it can give him a huge range increase through the range extension and additional jump.

Skill 3 (Glorious Shards) finds the enemy in range (Eyjafjalla S3 range) with the highest HP and generates a lightning storm around them for 4 seconds. Every 0.5 seconds, he attacks a random enemy inside the storm. This is Passenger's bread and butter. It's not a nuke, but it still does a lot of damage to a lot of enemies. The range is also huge, which gives him a lot of positioning flexibility. The main caveat is the targeting priority, so it's important to pay attention to his entire range so you don't misfire. The uptime isn't stellar, but players who want to spam him more can use his modules or external SP buffs.

Masteries

S3 is probably the only skill that most players will want to do masteries on. The gains aren't great, but the 20% attack buff does more than it seems like it does on paper - remember each of the hits chains. Also, the small decrease in charge time is still useful.

S2 has mediocre damage gains from masteries, but you will get an increase in uptime if you like to use it.

Modules

Passenger's modules give you a choice between quantity and quality on his skill.

Module X reduces the decay coefficient between chain jumps and increases the duration of his Slow to the same length as a Decel Binder supporter. This is a pretty big change, and you'll notice it immediately. The upgrades add an SP generation buff to talent 2. It's worth it to go to mod3, which gives him +0.25 SP/second. This still stacks with other SP sources like Mostima or Ptilopsis. This helps the uptime on S3 immensely if you want to make it more spammable.

Module Y eliminates the decay coefficient between chain jumps. The upgrades give talent 1 an additional damage increase, and mod2 adds an extra second to the duration of the buff. This module really leans into Passenger's role as a DPS unit, eliminating the damage decay in his attack chains. This lets him really milk the damage he does with skill 3. It also enhances skill 2 because you get 5 full-damage chains. The talent upgrades give him more time to whittle enemy HP down, which increases his DPS even more. Level 3 doesn't have much to offer, so I would just stop at level 2 for the extra duration.