• Moving Out
• Lovers in a dangerous Spacetime
• A way out (sensible topics though, violence/jail)

Phogs - simple puzzle platformer where you play an end of a 2 headed snake dog.

Lovers in a dangerous space time - share control of your spaceship to rescue cute animals. Take turns driving, manning the weapons or shields. Top down camera

Overcooked 1 and 2 - flammable chaotic cooking adventures. Top down camera.

Trine 1-4 are all pretty decent for some side scrolling puzzle action platforming.

Diablo 3 (on console as pc doesn't have local for some stupid reason) is a great action RPG.

Untitled Goose Game — Surprised it wasn’t mentioned.

Plateup! - Overcooked but a bit different in a way my wife and I find more enjoyable.

Crawl - Dungeon crawler type game with a twist

Death Road to Canada

Lovers in a Dangerous Spacetime

Battleblock Theater

Divinity Original Sin 1/2

Speed runners

For the King 1/2

Ship of Fools

Moving Out

Octodad

Pico park 1/2

Portal 2

Project Zomboid

Pummel Party - More of a party game tho

Sackboy A Big Adventure

Streets of Rogue

Struggling

Tools Up!

Trash Patrol

Ultimate chicken horse

Unrailed

Smash Brothers

It takes Two

Portal 2

Mario kart

TMNT: Shredder's Revenge

+1 for a tentative Overcooked suggestion but only if you think you and your cousin won't strangle each other in a high stress scenario. I can't play the game with my wife because she's too goofy and I'm too serious.

Bread and Fred looks like a lot of fun but I haven't actually played it yet.

Diablo 3 was fun playing couch co-op and reminded me more than a little bit of playing Gauntlet in the good old days.

• Castle Crashers
• Party Animals
• Overcooked (be ready to hate everyone you are playing with)

My family loves:

• Spelunky (sidescroller Indiana-Jones-themed platformer action-roguelite)

• Pikmin 3 (3D silly minion-bossing treasure hunting)

• River City Girls (sidescrolling beat-em-up with amazing soundtrack)

• Moving Out (3D "goofy physics" furniture-moving obstacle course)

• Kirby and the Lost World (3D action adventure)

• Stardew Valley (isometric farm sim)

All of these are fully coöp from the beginning.

If you wanna go retro at all you should check out UFO50. 50 very old school style games, many of which have 2 player modes.

WRT Secret of Mana - maybe play the first little bit yourself until you unlock a second character, then save and you two can start from there. Trust me.

Also, I know it's a side scroller, but Castle Crashers is great fun if you haven't tried it yet.

Pretty much any of the SNES era arcade ports: TMNT, Knights of the Round, Sunset Riders... Though maybe not all of them in one sitting.

That's all I've got off the top of my head. Happy gaming!

I think Stardew Valley has a Co-op mode? I'm not entirely sure

A friend and I had a little fun with Ultimate Chicken Horse. It's a goofy little Mario Maker type of game where you create levels and challenge each other to complete them but I think you can also run through them cooperatively.

Beyond all the other great suggestions, you can use the games browser on co-optimus to help narrow things down.

9 Parchments - Surprising gem with some depth and a lot of replay value. Reminds me a lot of the Harry Potter universe. We fire this up almost every weekend. I recommend setting friendly fire to 'self' instead, as it encourages actual cooperative play.

Teenage Mutant Ninja Turtles, Shredder's Revenge - this is just a hack n slash quarter eater, but it's mindless fun.

Most of my suggestions that I've been playing with my brother are already posted, so I'll just add 3 more I haven't seen yet:

• Human Fall Flat
• Enter the gungeon
• Vampire survivors

It takes Two (co-op puzzle)

Unravel 2 is a bit similar co-op puzzle game.

Rayman Legends. My literal childhood on the Xbox One.

I'm a massive fan of couch co-op, so here's some of my favourites:

Heave Ho: cooperative grabby-hand-based platforming

Moving Out: hectic moving simulator (get the first one, not the second)

Drink more Glurp: chaotic parody of a sports event, with grabby-hand mechanics, and wacky powerups to change things up

Wobbly Life: a kid-friendly open world game with jobs to do, and tons of secrets to find. Kinda like Roblox mixed with GTA. Pretty much endless fun.

Toasterball: it's like tennis, but with toasters.

Puyo Puyo tetris 2: Tetris

Ultimate Chicken Horse: Chaotic party platformer

Boomerangfu: brawler with boomerangs

Speedrunners: competitive high-speed platformer

• Cult of the Lamb
• Ship of Fools

Quite surprised these are not on here yet.

If you have a Nintendo Switch then Snipperclips is a really fun and cute co-op.

I've been playing Broforce with my buddy recently. Lots of shooting and blowing shit up platformer but pretty funny if you're into the classic action movies like Rambo, Commando, etc.

Gears of War if you want to branch into 3rd person cover shooter. (Gears 5 is the only one on steam I think.)

The Lego games usually have couch co-op I think and I've enjoyed those a lot with younger people.

Lovers in a dangerous space time: pilot a space ship with a friend and fight rogue constellations.

Ship of Fools:

Man the decks in a rogue-lite game where you fight sea monsters and find treasure

My list of coop games

• Baldur's Gate 3
• Broforce
• Castle Crashers
• Children of Morta
• Divinity Original Sin 2
• For the King
• "FORCED: Slightly Better Edition"
• Helldivers 1
• Lovers in a Dangerous Spacetime
• Rocket League

^ All of these are on Steam.

I’d like to add for switch: Snipperclips, Yoshi‘s crafted world and Pode

Some retro suggestions:

• Super Mario Galaxy 1/2 - it sounds like your cousin is probably too old for this but this has one of the best "younger sibling" modes. They help collect and shoot bits. Which ultimately doesn't matter much
• The Legend of Zelda 4 Swords Adventure - It's basically multi-player Link to the Past! Bonus points if you dig up a GameCube, link cables, and GBAs
• Kirby Super Star - a mix of competitive and coop games
• Super Mario World (any) - you take turns but if you don't go 6 levels without dying to his one it can be fun
• Halo 1-3 + ODST - Great classic shooters, the campaigns are customizable for difficulty, and a little kid probably loves alien shooters
• Borderlands 1/2 - Looter shooters with a sense of irreverance
• Dead Rising 2/Off the Record and 3 - Zombie hoard murdering with combo weapons
• The Simpsons '91, Battletoads, TMNT, Streets of Rage, etc - Beat Em Ups are great for multi-player

Rayman Legends is without a doubt my favourite coop platformer with fantastic music and great fluid controls. I strongly recommend it, it's tons of fun

Dark pictures is a pretty fun movie like Seires my friends and have have very much enjoyed!

There is also ultimate chicken horse (usually better with more than 2 people)

Castle crashers (Side scroller silly adventure game) Gang beasts is also pretty fun

Dont starve together

Vampire survivor

Castle crashers

And if you are into emulation most of ps2 era and before is designed for couch co op so hit the archives (top x lists on you tube makes it easy), depending on your cousins tastes

Streets of rage games are good fun

My kids and I have loved these:

Coop:

• Kingdom Two Crowns
• Wizard of Legend

Competitive:

• Stick Fight
• Bopl Battle
• Wand Wars

Idk if this is your speed, but my kid and wife and I love playing the Lego games that are based on movies - Star Wars, Indiana Jones, Avengers, etc.

Very couchable and can be challenging if you try to get 100% on each level.

Apologies if I misread the ask!