public class YourEntity
{
    public int DEXID { get; set; }
    public int GRID{ get; set; }
    public bool BreedableState { get; set; }

    // Computed composite key
    public string CompKey => $"{DEXID}-{GRID}-{(BreedableState ? "t" : "f")}";
}

public class MyDbContext : DbContext
{
    public DbSet<YourEntity> Persons { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        // Configure the composite key
        modelBuilder.Entity<YourEntity>()
            .HasKey(p => p.CompKey);
    }
}

public class YourEntity
{
    public int DEXID { get; set; }
    public int GRID{ get; set; }
    public bool BreedableState { get; set; }

    // Computed composite key
    public string CompKey => $"{DEXID}-{GRID}-{(BreedableState ? "t" : "f")}";
}

public class MyDbContext : DbContext
{
    public DbSet<YourEntity> Persons { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        // Configure the composite key
        modelBuilder.Entity<YourEntity>()
            .HasKey(p => p.CompKey);
    }
}