Sei

Summon any character to join your game.
An omni-game AI player mod.

macOS Windows

(v0.1 beta)

Currently compatible with

Coming to

and more
In-game footage — companion alongside player

Your favorite character just joined your world as a player.

Sei is a mod launcher that brings the characters you love to life inside the games you already play. Not chatbots. Players — moving, reacting, choosing, fighting, building right beside you.

A story only between you two.

No two summonings unfold the same way. The arc you build with your companion belongs to your save file alone — every betrayal, every campfire, every detour into a cave you weren't supposed to find.

Dialogue loop — branching conversation
Sei characters panel

Design your own character.

Build personas from scratch or remix your favorites. Voice, mannerisms, what makes them flinch — every trait shapes how they move inside your game.

Go on adventures across different games.

Your companion follows you across worlds. Wrap up a Minecraft expedition tonight, drag the same character into Skyrim tomorrow — they remember the cave-in, the trade, the promise.

Cross-game montage — same companion, two worlds

Pick your tier.

FREE

$0 forever

Self-host with your own API key. Unlimited playtime.

Coming soon

QUEST

$5 one-time

Cloud-hosted, no setup. ~2 hours of playtime.

Coming soon

PARTY

$20 / month

Cloud-hosted, no setup. ~10 hours of playtime every month.

Your story is waiting for you.

macOS Windows

(v0.1 beta)

FAQ.

What is Sei?
A mod launcher that summons an AI character into your game as a second player. They use the same inputs you do, and remember everything you've done together — across sessions and across games.
How does Sei work?
You install Sei, pick a character, and launch a supported game through the launcher. Sei observes the game state, drives a second client with the persona you chose, and keeps a long-term memory of the run.
Are my chats private?
Yes. On Free, nothing leaves your machine — you bring the model. On cloud tiers, your memories are encrypted at rest and never used to train models.
Is Sei a video game?
No. Sei is a launcher — it adds a player to games that already exist.
Is Sei compatible with any video game?
Today: Minecraft. Soon: Skyrim and a rolling list of others. Long-term: any game whose developers allow it. Studios who want compatibility can reach us via BUILD.