I’ve been having this discussion/argument with another person I play DND with via Discord. He’s more of the rules type, saying that Volo’s states that unless you were an evil or Fallen Aasimar an Aasimar PC had to have a deva; I like to be more creative with my character creation and say that devas aren’t a requirement for an Aasimar and that it could open up more interesting rp or backstory opportunities if one played an Aasimar PC without a deva or never meeting their deva, just being blessed with divine ability or inheriting it through lineage.

Part of the reason I’m of this opinion is the one time I’ve played a Scourge Aasimar I thought it interesting if part of the reason he was a hermit was because the first time his Burning Light triggered it hurt and scarred his adopted brother so he ran and lived in seclude as to not hurt anyone else with it, so now he looks for a way to control it, using concoctions and potions to keep it under control. 

I am interested to hear what you all would have to say about whether a non Fallen Aasimar is required to have a deva or not.

This is solidly an "ask your DM" question. There are zero game balance implications here. It's purely going to depend on the lore of the world they are creating.

That said, per RAW, your friend is mostly right. Volo's does say that an aasimar that's not fallen has a link to an angelic being. Though it says "usually" a deva. So, it doesn't have to be a deva, necessarily. But given the space Volo's devotes to this guide and how the DM could play it, it's envisioned to be a big factor in the assimar's life. It also says the guidance you get from said angelic being only really comes in dreams, so it seems perfectly reasonable that you wouldn't have met yours. Maybe your Deva is a slacker and never reached out to you, or imprisoned and can't reach out, or thinks that so far you've been doing just fine without guidance, so they haven't bothered. So if a player came to me wanting to have an assimar without a deva I'd try to work with the player on why they didn't have one. Or I might just decide for them, and then leave it to the player if they want to pull on that thread. Like you find out over the course of the campaign that the deva that was supposed to be your guide is trapped in hell somewhere. Do you try to save them or decide you're doing just fine without Ms. goody-two shoes looking over your shoulder. It could be a pretty fun character side quest.

So, to reiterate, ask your DM. Their opinion is the one that will matter.

Is the person with whom you're discording DMing the game in which you intend to play this Aasimar, or are they simply another player who thinks they know everything about Aasimar and is trying to "teach" you a thing or two?  In the latter case, the other player's opinion is immaterial.  If they're the DM, player concept including backstory are contingent upon DM approval.  Some DMs are loose or flexible with lore, and some even get inspired by what a player may want to contribute to new grounds in world building.  Others have a very specific way things work, especially powers derived from extraplanar entities.  So it comes down to what you want an Aasimar to be, what if any relationship it has with the Deva or whatever source of its power to be, and your DM conceding that this would function in their game world.  

Some folks play or want Aasimar to be like Tieflings.  That is, re: Tieflings, in most game worlds they seem to serve little "purpose" in their being other than their Tiefling features expressing their family's prior dealings with Infernal forces at some point in the family tree.  With that origin, the Tiefling then goes about finding their own purpose in life. 

Some folks want Aasimar to be the Celestial equivalent of Tieflings, which seems to be your case too.  I can see that desire for symmetry, but despite Great Wheel orientation, as written all plane touched beings aren't simply Great Wheel geometry corresponding equivalents.  Aasimar are described as having a role as an agent in something involving the forces that imbued them with their powers, there's purpose to their creation.  And the write up provides tables to determine the origin and help create that purpose.

That said, and sticking to the write up as written, what's the big deal about having a Deva?  As written, the deva doesn't have any governance over the Aasimar (provenance is another story, rimshot).  The Aasimar is largely what spiritualist would call a "mystery" to the Aasimar.  Communication comes from visions and dreams, lot of collaborative potential between a player and DM on the same note.  Beyond that, it's on the player's creativity what if anything the Aasimar does with these messages.  The Deva isn't a control on the Aasimar, it's a conduit trying to guide the Aasimar, but open to a lot of interpretation or just ignoring.  

I'd have no problem with an Aasimar not heeding or not recognizing the guidance its receiving, other DMs may see it functioning differently.  I mean taking a Scourge Aasimar and giving them a back story akin to any number of the X Men isn't such a creative leap away from the Aasimar as written (Nightcrawler could be particularly inspirational if you want to address the whole reconciling with a higher power route).  Again, you could have a DM that's fine with an Aasimar that can't or won't "listen."  But you may have a DM who could see consequences for a character whose Deva sees them as "sitting out" the cause.  It's really not different from say a character adventuring despite their noble station, etc.  

I guess I'm saying I don't see the need for a conflict.  If you have a DM who sees an Aasimar choice as somehow license to control or dictate the PC's behavior, that probably requires you two to look over the description where that license is in fact not granted, or maybe realize that this DM has some pretty strict interpretations as to how certain aspects "must" be played. If it's just a player telling you what you need to do, they're not part of the equation.

Volo's Guide is a bit problematic reputation as being too absolute about various things.  Given that Volo had a reputation as being a blowhard who didn't know nearly as much as he thought he did back in 2E, I'd be inclined to treat the info in that book as an in-universe source that's not necessarily 100% accurate.

Quote from 6thLyranGuard >>

Volo's Guide is a bit problematic reputation as being too absolute about various things.  Given that Volo had a reputation as being a blowhard who didn't know nearly as much as he thought he did back in 2E, I'd be inclined to treat the info in that book as an in-universe source that's not necessarily 100% accurate.

Yeah, but the way the deva : aasimar dynamic arguably in "volo's voice" corresponds with Baker and Mercer's voice in the Aasimar write ups for Eberon and Wildemnot.  It can be neglected but that's three reflections of the same trait that I'm almost guaranteeing it'll appear in MMM, to be done with as the table will since it's not really a trait compelling the PC to actually do anything.  It's more akin to Elves don't really dream, Warforged have their sleep mode, when Aasimar's rest, the DM can introduce visions, which can be ignored.  Maybe the Aasimar is like Rust Cole in True Detective and thinks there's some meaning in every flock of crows and angle of sunlight, but cant' really get their head around it.

There are other celestials than devas, I don't see why aasimars should be limited in such a way.  It's perfectly fine if someone wants to play an aasimar character like that but they shouldn't be forced to if they have another idea for how they want to do things.

Sure, I'm very much of the "you don't gots to" pending what the DM has bothered to think of their cosmology.  But really the OP's "I want to be more creative" stance (to play basically an X Men or otherwise familiar back story) to me seems to be missing the actual creative collaboration potential if they actually considered what the text actually offers, a potential which is actually pretty cool.  Seemed like the OP was being brow beaten by another player re: controls placed on the race. Whereas to the letter of the text, there's no such thing, but rather some cool opportunities.

play it however you want.

Not 5e, but if I remember correctly in the novel, Godborn, the aasimar is a type of deva.  When the aasimar (can't remember his name) first meets Erevis's son, he says something along the lines of 'some call me a deva, some an aasimar'.

I completely forgot aasimars had that whole deva thing because none of the aasimar PC I've encountered ever did anything with it lmaooooooo.