The first encounter with Starwalker can be found in the room where you are chased by a bird that looks vaguely like Starwalker, though it is simply a shameless ripoff of the original character. Once you ring the bell to destroy the fraud's wings, walk back to where you entered the room to meet Starwalker.
Once you meet Starwalker in this room, Starwalker will now appear throughout the rest of the entire game. If you did not speak to him here, you will have an inferior save file that does not contain Starwalker.

Chapter 1's second Starwalker appearance is in the Card Castle Dungeon, on the far right side of the room with the Goulden Sam Cages.
He is not happy about this jail.

Starwalker's first appearance in Chapter 2 is unmissable, provided you met him in Chapter 1. When Lancer and Rouxls enter your inventory, Starwalker will also enter your inventory.
He will chill in your Key Items for most of the Chapter, in the top left corner.

Starwalker will appear in the room where you can give a present to one of the main cast members. He will follow you around for the remainder of the time you spend in the room before returning to your Key items.
The 6 Heroes.

Starwalker's final appearance in Chapter 2 is right before the Giga Queen fight, where he appears, along with all your other recruits, to upgrade the Thrash Machine
Truly a quote to live by.

Starwalker's first appearance in Chapter 3 can be found in the room with the first save point. Head to the right where Tenna is grooving, and then backtrack to the far left.
He is the true main character.

Starwalker appears again in the Goulden Sam room during the TV World section of Chapter 3.
The prison still pisses him off.

After the Rouxls Kaard fight, Lanino and Elnina decide to form a new polycule with Starwalker.
They can all share the sky together.

After unlocking the fast travel doors, head back to the TV World entrance and keep heading left. On the far left of the room with the prize balls, you can find Starwalker.
He was the Star

walker.

Starwalker's final appearance in Chapter 3 is during the Tenna boss fight. If you unlocked the Monster Movie minigame in the A-Rank Room in the Green Room with 500 Points, new variants will appear during Tenna's fight. During the version where you must protect Ralsei, Starwalker can be seen helping Kris with the rope holding Susie up.
Starwalker is a true hero.

Starwalker does not appear for most of this Chapter. His first appearance is in The Second Sanctuary, in the room featuring a bunch of statues. He has somehow become a statue as well. One of the philosophers seems transfixed by how original he is.
How could this have happened...

Luckily, Starwalker does not stay petrified for long. He reappears in The Third Sanctuary, helping out the stack of all your recruits.
It would fall apart without him...

Credit to the Deltarune Wiki for the Chapter 2 Screenshot showing Starwalker entering your inventory featured in this guide, because I was too lazy to play through the short opening section of Chapter 2 again. Link: https://deltarune.wiki/w/Starwalker

You should also check out this video: https://www.youtube.com/watch?v=JfapcR0dw4Q
It wasn't made by me, but it helped me find some locations I missed, and basically served as inspiration for me making this "Guide".