FOR CONTROLLER PLAYERS:
start a new run and add a co-op player.. should work

FOR KEYBOARD PLAYERS:
If you have the Debug Console enabled, You should run the following command:
"Addplayer (ID)"

Valid ID's for Non-Mod characters

---Un Tainted---
0/none = Isaac
1 = Magdalene
2 = Cain
3 = Judas
4 = Blue Baby/???
5 = Eve
6 = Samson
7 = Azazel
8 = Lazarus
9 = Eden
10 = The Lost
13 = Lilith
14 = Keeper
15 = Apollyon
16 = Forgor
18 = Bethany
19 = Jacob (singular char.)
20 = Esau (singular char.)

---Tainted---
21 = T Isaac
22 = T Mag
23 = T Cain
24 = Dark Judas
25 = T ???/Blue Baby
26 = T Eve
27 = T Samson
28 = T Azazel
29 = T Lazarus
30 = T Eden
31 = Ghost of late night skill issues- i mean T Lost
32 = funny preggo demon lady (T Lilith)
33 = T Keeper
34 = T Apollyon
35 = T Forgor
36 = T Beth
37 = T Jacob (+ Dark Esau, or whatever mysterious suspicious mod you used to replace him.)
38 = Lazarus2 (A.K.A Dead Laz)
39 = players.xml is telling me this is T Jacob again so i guess it is?
40 = The Soul ( Forgotten's Soul)

Thank me later. I guess