Solarc

Yeah, I'm certain I'm doing that part of the process correctly. I'm changing the nature from Lax to Serious and then the characteristic from "Mischievous" to "Likes to run," the former works as intended while the latter keeps showing as "Loves to eat."

I've done this countless times before using gen VI and VII saves with no problem: I switch to a neutral nature, set all of my IVs to be the same value (in this case 0), and then reroll the EC until I get the characteristic I want. Most recently I got "Likes to run" for Sprigatito, but upon loading the save it shows as "Loves to eat" in-game, I made sure to double check. The issues reported in these recent threads sound pretty similar to mine:

Exception Details: System.ArgumentOutOfRangeException: Specified argument was out of the range of valid values. Parameter name: length at System.ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument argument) at PKHeX.Core.SAV3Colosseum.SetChecksums() at PKHeX.Core.SAV3Colosseum.GetInnerData() at PKHeX.Core.SAV3Colosseum.GetFinalData() at PKHeX.Core.SaveFile.Write(ExportFlags flags) at PKHeX.WinForms.WinFormsUtil.ExportSAV(SaveFile sav, String path) at PKHeX.WinForms.WinFormsUtil.ExportSAVDialog(SaveFile sav, Int32 currentBox) at PKHeX.WinForms.Main.ClickExportSAV(Object sender, EventArgs e) at System.Windows.Forms.ToolStripItem.RaiseEvent(Object key, EventArgs e) at System.Windows.Forms.ToolStripMenuItem.OnClick(EventArgs e) at System.Windows.Forms.ToolStripItem.HandleClick(EventArgs e) at System.Windows.Forms.ToolStripItem.HandleMouseUp(MouseEventArgs e) at System.Windows.Forms.ToolStrip.OnMouseUp(MouseEventArgs mea) at System.Windows.Forms.ToolStripDropDown.OnMouseUp(MouseEventArgs mea) at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks) at System.Windows.Forms.Control.WndProc(Message& m) at System.Windows.Forms.ToolStrip.WndProc(Message& m) at System.Windows.Forms.ToolStripDropDown.WndProc(Message& m) at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam) Loaded Assemblies: -------------------- mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089 C:\Windows\Microsoft.NET\Framework64\v4.0.30319\mscorlib.dll PKHeX, Version=22.2.4.0, Culture=neutral, PublicKeyToken=null D:\PKHeX\PKHeX.exe System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089 C:\WINDOWS\Microsoft.Net\assembly\GAC_MSIL\System\v4.0_4.0.0.0__b77a5c561934e089\System.dll System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089 C:\WINDOWS\Microsoft.Net\assembly\GAC_MSIL\System.Windows.Forms\v4.0_4.0.0.0__b77a5c561934e089\System.Windows.Forms.dll System.Drawing, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a C:\WINDOWS\Microsoft.Net\assembly\GAC_MSIL\System.Drawing\v4.0_4.0.0.0__b03f5f7f11d50a3a\System.Drawing.dll System.Configuration, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a C:\WINDOWS\Microsoft.Net\assembly\GAC_MSIL\System.Configuration\v4.0_4.0.0.0__b03f5f7f11d50a3a\System.Configuration.dll System.Core, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089 C:\WINDOWS\Microsoft.Net\assembly\GAC_MSIL\System.Core\v4.0_4.0.0.0__b77a5c561934e089\System.Core.dll System.Xml, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089 C:\WINDOWS\Microsoft.Net\assembly\GAC_MSIL\System.Xml\v4.0_4.0.0.0__b77a5c561934e089\System.Xml.dll PKHeX.Core, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null PKHeX.Core, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null System.Memory, Version=4.0.1.1, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51 System.Memory, Version=4.0.1.1, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51 System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a C:\WINDOWS\Microsoft.Net\assembly\GAC_MSIL\System.Runtime\v4.0_4.0.0.0__b03f5f7f11d50a3a\System.Runtime.dll System.Runtime.CompilerServices.Unsafe, Version=4.0.4.1, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a System.Reflection, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a C:\WINDOWS\Microsoft.Net\assembly\GAC_MSIL\System.Reflection\v4.0_4.0.0.0__b03f5f7f11d50a3a\System.Reflection.dll System.Runtime.InteropServices, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a C:\WINDOWS\Microsoft.Net\assembly\GAC_MSIL\System.Runtime.InteropServices\v4.0_4.0.0.0__b03f5f7f11d50a3a\System.Runtime.InteropServices.dll System.Runtime.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a C:\WINDOWS\Microsoft.Net\assembly\GAC_MSIL\System.Runtime.Extensions\v4.0_4.0.0.0__b03f5f7f11d50a3a\System.Runtime.Extensions.dll Newtonsoft.Json, Version=13.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed System.Numerics, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089 C:\WINDOWS\Microsoft.Net\assembly\GAC_MSIL\System.Numerics\v4.0_4.0.0.0__b77a5c561934e089\System.Numerics.dll System.Runtime.Serialization, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089 C:\WINDOWS\Microsoft.Net\assembly\GAC_MSIL\System.Runtime.Serialization\v4.0_4.0.0.0__b77a5c561934e089\System.Runtime.Serialization.dll System.Data, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089 C:\WINDOWS\Microsoft.Net\assembly\GAC_64\System.Data\v4.0_4.0.0.0__b77a5c561934e089\System.Data.dll PKHeX.Drawing.PokeSprite, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null PKHeX.Drawing, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null System.ValueTuple, Version=4.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51 C:\WINDOWS\Microsoft.Net\assembly\GAC_MSIL\System.ValueTuple\v4.0_4.0.0.0__cc7b13ffcd2ddd51\System.ValueTuple.dll PKHeX.Drawing.Misc, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null -------------------- User Message: An unhandled exception has occurred. You can continue running PKHeX, but please report this error.

I remember doing this once before with an USUM save and I don't think I had any trouble then, any idea what I could be doing wrong?

How hard would it be to get this info by looking through the game files?

I noticed you're able to change your starter's encryption constant when clicking on the Hall of Fame section of an USUM save (I'd imagine it's the same for SM), which means the game keeps track of it for whatever reason... Why? I couldn't find an answer to this anywhere.