Because I know there are a few Simmers who are interested in having Grim as a playable character, this is a step by step guide to making that happen using either just cheats or with mods.

Cheats only

You're going to have kill a sim. Obviously this is a lot easier if you have the MCCC mod installed, but if not then I'd say the most fool proof method would be to set lifespan to short and then either create an elderly Sim or age up one of your existing Sims to elder. You can obviously try and kill them in other ways, but my understanding is those sometimes aren't as effective or easy as you'd think.

Okay, so you've got your dead Sim, and Grimmy has shown up to reap their soul. Wait for him to do his job, then hit pause (so he doesn't disappear on you whilst your typing in cheat codes), and press Ctrl-Shift-C to open the cheats tab. Once you have the cheats tab open type 'testingcheats true'* and then hold down shift whilst clicking on Grim and select 'add to family'. Congratulations you now have Grim as a household member.

If you want to do more than just have your Sim be BFF's with Grim, and let's be honest that's probably why most of us have Grim in our Sims families, then you'll need a few more cheat codes. In the cheats tab again type 'cas.fulleditmode', then shift+click on Grim again and you should get an option that says 'Edit in CAS'. Select that and you should be able to edit whether or not he can get a sim pregnant, or get pregnant himself (whatever floats your boat).

So potential love is in the air, and your Sim is ready to start falling in love, dating, getting married and having adorable Reaper babies. Not so fast, there are limitations on your romantic actions with Grim. Without cheats you can still get the romance bar with him pretty high, but you'll be limited to interactions like 'flirt, kiss cheek, hold hands, embrace, compliment appearance, etc'. If he's in a confident mood, feeling flirty, or it's a particular holiday then you might get some extra interactions such as 'suave kiss' or 'passionate kiss'. The 'ask on date' option will appear eventually as well, but if you try and go on date with him the date will just automatically end. To override this, including no option to Woohoo or try for baby, you're going to have to use cheats to remove Grim's reaper traits. Open the cheats tab again (ctrl+shift+c), then type in 'traits.remove_trait GrimReaper'. This will remove Grim's floaty effect and his reaper voice, but you will be able to do pretty much anything with him that you would be able to do with any other Sim, including all romantic actions.

Once you've achieved your goal(s) with Grim you can give him back his reaper traits if you wish. Open the cheats tab, then type 'traits.equip_trait GrimReaper', hit enter, and he'll be back to his usual Grim self.

(*without the quotation marks, obviously)

With Mods

Have MCCC installed, select any Sim on a lot, click on them and then click on the MCCC icon and go to 'Maintain Sim', then choose 'Kill Sim' and pick your method of death. I find pretty much anything except for death by lightning strike or fire to work best.

Now Grim has shown up, again wait for him to do his job, then shift+click add to family. This should work automatically with MCCC installed, but if it doesn't then just use the 'testingcheats true' method as described in the cheats only portion above.

Unlock all actions for Grim by downloading and installing the Sims 4 Reaper Actions Unlocker mod by Gauntlet101010*. You can google it, it should be a link to a deviant art site. Using this mod you can have Grim go on a date, get married, woohoo, try for baby, and go swimming without having to type out cheat codes, and he'll maintain his floaty smoke and reaper voice traits as well.

If you also want to make Grim a soul reaper again you can also download and install the Soul Reaper Career mod also by Gauntlet101010*. Just follow the instructions on how to use the mod, and bear in mind that the mod makes your Grim a soul reaper, not 'The' soul reaper. He'll only reap the souls of those who have died on a lot when either he, or one of his household/family members are present (the game does still generate an NPC Grim Reaper).

And that's pretty much it. Any questions feel free to ask, and Happy Grimming!

(* a quick note on both of the Reaper mods; they do seem to cause exception error notices to pop up occasionally with MCCC installed. Apart from the odd annoyance though I haven't really noted anything else like certain mod actions not working).

If you only want Grim's babies & still want him to remain as an NPC (with thanks to Harley over at reddit for the additional advice):

Move Grim out of the family after you've done what you needed to with him, and then click on the mailbox, select hide household and select Grim's household. Now he'll be back to regular game Grim. Please note I haven't personally tested this so I don't know if it still works, I assume it does, and it makes sense that it would.