Biomutant's open-world environment is filled with 23 different secondary characters with their own unique quests and special unlockables. Finding them all will earn you the “Social” trophy/achievement, and each of these secondary characters is important in its own way. Certain characters will help you unlock unique mounts like the flying Batnam-nam, get character customization services, or earn special vehicles like the airship. These characters are incredibly easy to miss, even if you've also thoroughly explored the area of the map they're in.
I recommend avoiding tracking them until you've pretty much accomplished most of your goals. You will find most of these secondary characters naturally by exploring. When you've found a bunch of them, it's a good idea to go through the list and mark areas of the map that need further investigation - that way you'll only have to hunt down 3-4 instead. of a full set of 23. That's my advice anyway.
More biomutant guides:
Best Weapon Slot | Best Weapon Location | Where To Get Flying Mount & Blimp | How Tribe War Works | How to customize your character
All Secondary Character Locations | Map Coordinates Reference Guide
Below you will find all side character locations listed in their sector with approximately [X,Y] coordinates. You will find the sector name and coordinates in the lower right corner of your map.
boom: Sector 7C X: -106800, Y: -48875
Chu-Chu: Sector 9I X: -216738, Y: -271812
Chubsa: Sector 8E X: -199988, Y: -52938
Gil: Sector 11G X: -346925, Y: -188906
Gizmo: Sector 5C X: -1644, Y: -23594
Goop: 4D sector X: -10262, Y: -24500
Sip: Sector 4E X: -32419, Y: -63781
Honki: Sector 7G X: -146394, Y: -170344
Juju: Sector 6G X: -59956, Y: -197531
clutter: Sector 7E X: -119612, Y: -67313
Click : 5D sector X: -32581, Y: -47219
Lobo: Sector 9J X: -236644, Y: -342312
Piece: Sector 3F X: -90637, Y: -138562
Maya: Sector 7K X: -111175, Y: -399219
Mito: Sector 5J X: -27613, Y: -323062
Moog: Sector 6C X: -65050, Y: -24406
Noko: Sector 6J X: -59456, Y: -341125
Nova: Sector 10E X: -275050, Y: -90094
Expired: Sector 8G X: -166144, Y: -162031
Galet: 7D sector X: -121300, Y: -11781
Sol Slenderstiff: Sector 11K X: -331019, Y: -378250
Garnish: Sector 6F X: -95050, Y: -14188
Whistling: Sector 12F X: -376644, Y: -118281
