v.2.12.
Replaced Rgon check higher than Rep+ check, so in case you're using nightly build, you'll have Rgon synergies.
Fixed setting menu in ImGUI becoming desynchronized when changing settings in MCM.
In case you have Rgon, mod now will only reload config upon changing saveslot, instead of doing it upon every run.

Neptunus + Lasers:
(Rgon) Now only removes light from related effects (impacts, anti-gravity swirls, brimballs), since color of laser-emitted-light is now correct.

Neptunus + Technology + Chocolate Milk:
(Rgon) Fixed that lasers were fired with their sprite changed, even when having custom sprites disabled.

Neptunus + Epic Fetus:
(Rep(+)/Rgon) Fixed error, when playing as T.Lilith and baby was retracted before rocket landed.

v.2.11.

Neptunus + Tech X:
(Rep(+)) Now charge is only lowered instead of being zeroed out.

Neptunus + Dr. Fetus:
(Rep(+)/Rgon) Found out tears were also spawned like for Epic Fetus synergy, which is now fixed.

Neptunus + Epic Fetus:
(Rep(+)/Rgon) Because of some changes to Forgotten+Neptunus, this didn't decrease charge. Fixed.

Neptunus + The Forgotten/Berserk!:
(Rep(+)/Rgon) If has Epic Fetus, always decreases charge after swing.

v.2.10.
Changed icon, so it shows that mod works with Rep+

Neptunus + Mom's Knife + Ludovico:
(Rep(+)/Rgon) Fixed that synergy stopped working when having multiple players/familiars. Thanks to @Dr. Coitus for discovering this.

Neptunus + Technology / Brimstone / Tech X:
(Rep(+)/Rgon) Light emitted by lasers now has corresponding color. Though, I couldn't change color of light emitted by laser related effects (impacts, anti-gravity swirls, Forgotten's brim-ball).

Neptunus + The Forgotten/Berserk!:
(Rep(+)/Rgon) Changes to behaviour (again).
⠀• Now, once again, Neptunus charge will only be decreased once you finish attack (throw bone, or don't charge throw attack once swing is complete). Because of that, there is no need for +15% Neptunus charge to bone throw splash, and so it was cut.
⠀• In other aspects it mostly works like before.
⠀• Actually, it was rewritten pretty much, to work as correctly as possible with all effects, but you probably won't notice.

v.2.9.
Fixed incompatibility with Ludovico Laser Fix. Thanks @Dr. Coitus for reporting this.

Neptunus + Epic Fetus:
(Rep(+)/Rgon) Charge is now strongly reduced on each* rocket hit, instead of being reset after all rockets hit.
*each subsequent rocket. Rockets that fall on same frame (e.g. from Loki's Horns) will not cause extra decrease of charge, while ones that do fall one after one (e.g. Mutant Spider) will reduce charge further with each hit.

I'd really appreciate some feedback on this change, because I myself feel very conflicted about it. On one hand, all subsequent rockets are just parts of same attack, and so they should receive same effect as first rocket. On other hand, that sounds somewhat op, and decreasing charge after each strike just felt somewhat right to me.

Neptunus + The Forgotten/Berserk!:
(Rep(+)/Rgon) Now both swing AND throw attacks decrease Neptunus charge, instead of Neptunus charge being zeroed out, after attack was totally finished.
⠀• Due to the fact, that technically means throwing bone attacks just can't be "fully" charged, bone throws perform as if they had +15% to their charge, not exceeding max. Same applies for Bone+Epic Fetus Rockets.
⠀• When having autoshoot (Soy Milk/Almond Milk/30+ tears), throw attacks will always create fully charged splash, regardless of your current charge.
⠀• Brim and Lung synergies heavily decrease Neptunus charge.
⠀• Each Cursed Eye swing decreases Neptunus charge.

v.2.8.
Updated to work with Repentance+ 1.9.7.7, as this version of Rep+ broke mod.
This means nothing as Nicalis can break it again.

Added menu in ImGui for Repentogon.

Neptunus + Brimstone:
(Rep(+)/Rgon) when firing, charge gradually decreases, instead of being zeroed out instantly. Charge stops decreasing, if you cancel your shot by starting to charge new beam.
(Rep(+)/Rgon) fixed that charge was consumed way faster than intended in Nep+Brim+Soy synergy.

Neptunus + Spirit Sword:
(Rep(+)) now neptunus-spins reflect bullets. Done in a sort of workaround way, so it works different from vanilla.

Neptunus + Monstro's Lung + Chocolate Milk:
(Rgon) fixed that it always fired minimum dmg tears, unless you had Soy Milk or 30+ tears stat.

Neptunus + The Forgotten/Berserk! + Brimstone:
(Rep(+)/Rgon) Now duration of brim-balls is also increased (up to 3x), but size increase was lowered a little (so it's more consistent to current Brimstone synergy).

Runned metadata stripper on pngs, cutting mod size a ₗᵢₜₜₗₑ bit.

v.2.7.
Fixed oversight in non-Rgon version, that caused mod not to work. Thanks to @CullyDC for reporting.

v.2.6.
General:
(Rep/Rgon) Found out familiar's damage multiplier is already included in functions like Player:FireTear(), when you mark that familiar as source, thus multipliers were used twice. This is now fixed.

Neptunus + Tech X:
(Rep) increased ring's width bonus.
(Rep/Rgon) Tech X's dependance on tears stat in laser-ring radius and damage multiplier is now calculated fully correctly.

Neptunus + Monstro's Lung:
(Rep/Rgon) Lowered tears/mini-bombs average speed.

Neptunus + Forgor + C Section:
(Rep/Rgon) Fetuses now:
1) don't use player's charge;
2) their swings don't demand player to have any level of Neptunus charge to fire swing tears;
3) tears from their swings always do 0.6x damage;

v.2.5.
Neptunus + Epic Fetus:
(Both Rep/Rgon) changed formula for number of tears. Now, Neptunus' charge affects more, and your tears stat affets it less; final result is additionally wrapped under square root, so it wouldn't grow too big.

Neptunus + Technology + Monstro's Lung:
(Rep) fixed error that appeared, when familiar (Incubus and alike) fired attack.

Neptunus + Spirit Sword:
(Both Rep/Rgon) limit, below which transformations aren't performed, is now set higher; consumption of each transformation is now way lower.
(Rgon) new Repentogon functions made possible better transformation of swing to spin attack: bullets are now reflected and size of attack's hitbox is now same without additional enlargement of sword's slash sprite.


Better compatibility with JamesB456 mods:
Epic Fetus Synergies

• Epic Fetus + Ludo: now Neptunus charges correcly; each explosion lowers charge, instead of resetting it when there are no rocket targets left.
  
• Epic Fetus + Soy: Neptunus charges way slower (due to how high your tears stat will be, it won't be a problem); each explosion lowers charge, instead of resetting it when there are no rocket targets left; number of tears created by explosions is lowered, but their dmg mult is increased.

• Spirit Sword + Sprinkler & Spirit Sword + Shade: aren't affected now (were consuming player's charge previously).

v.2.4.
Neptunus + Monstro's Lung
(Rgon) changed synergy.
⠀• Old: fires up to 2x tears.
⠀• Current: changes Monstro's Lung to more tear-like behaviour, making Neptunus work with it like it would with tears.

Neptunus + Monstro's Lung + Dr.Fetus
(Rep/Rgon) Added correct behaviour for synergy. Now Neptunus will fire additional mini-bombs instead of continuing firing tears.

Neptunus + Monstro's Lung + Technology
(Rep/Rgon) changed synergy.
⠀• Old: increases length and width of lasers, depending on Neptunus charge; upon shoot Neptunus charge resets to 0.
⠀• Current: chance to spawn additional arcs; chance increases with Neptunus charge; upon shoot Neptunus charge is heavily lowered.

Misc
(Rep/Rgon) Mod now changes sprites for Haemolacria tears, instead of recoloring them.
(Rgon) Lasers from Technology 2 and Tech.5 should no longer have their sprites affected.

Final notes
Regarding Rgon Monstro's Lung synergy. Though it was possible to just give soy-milk weapon modifier, allowing autoshoot, it was too buggy, so I sticked to the old way, in which I dealed with Tech X and Technology + Choc Milk (a.k.a. remade attack myself, trying to fully replicate it with all the knowledge I could gather).
Also, I couldn't make anims work, though I've tried using Rgon's Weapon:PlayItemAnim(), sorry.

v.2.3.
Neptunus + Monstro's Lung
(Rep/Rgon) synergy update.
1) Attack is now closer in behaviour to vanilla one (should be nearly indistinguishable).
2) Instead of constantly firing up to 14 tears at full Neptunus charge in front, mod now tries to imitate your attack as much as possible. That includes:
⠀•⠀Additional tears in bursts from Inner Eye, 20/20, Lung's copies and else;
⠀•⠀Additional bursts from Mom's Eye, Loki's Horns, Eye Sore and Wiz;
⠀•⠀Attack direction change from Wiz.
3) In case attack wasn't fully charged, mod now will release attack, depending both on Neptunus and normal charge.

Final notes
Without Repentogon it's problematical to detect, whether Mom's Eye, Loki's Horns or Eye Sore should fire (in case of last item, it's also problem to know how much), and due to many possibilities, I didn't even try that.
As result, without Repentogon mod, depending on game's formulas, just decides itself, whether it should fire these, so don't be surprised if your fully double-charged attack released less tears backwards/in sides than you should have. It's just because game and mod got different random.

However, even with Repentogon, I can't know in which direction Eye Sore should fire, so it's just randomized.