I think Chapter 1115 has shown us some hints what happened during the void century.
First off, the letter D is a symbol created by Joyboy for his crews. Later they realized they losing the war, so they add the letter D into their name and pass it down generation hoping someone in the future will rise up against the WG. Lily left the crew too early like Vivi did and realized it too late, so she wrote it in a letter to pass it down to her descendant.
There were betrayal in the Joyboy crews, it was hinted by Doflamingo.
I think the appearance of Miss Golden Week in Chapter 1115 kinda hinted that whoever had the yami devil fruit is the betrayer of Joyboy during the void century.
The clues are from her colour traps abilities. Yellow of Laughter indicate Sun God Nika Joyboy where Black of Betrayal indicate the betrayer having yami devil fruit.
The reason of betrayal is because of a simple argument which they are completely opposed of what the other said.
There is a hint from the argument between Shanks and Buggy. Shanks and Buggy have complete opposite personality. Shanks is positive of being brave, selfless, truthful whereas Buggy is negative of being coward, greedy, deceitful.
Buggy hated Shanks so much that make him wanted to kill him.
Since Shanks and Buggy argue about South and North pole, I assumed the argument between Joyboy and betrayer are East and West. If the Earth were rotating from west to east and if one were to travel around the Earth in one circle, in which direction will be faster. This is probably why the red line and grand line exist because red line represents the start and finish line, and grand line represents the race track. whoever reached the finish line first were to be declared winner.
They had a fight and led to one leaving the crew.
Luffy and Usopp are exactly Shanks and Buggy respectively. They share the same personality.
I Think the hint of Foxy appearance in chapter 1115 support my idea of Joyboy defeated the betrayer resulting in leaving the crew. Looking closely Foxy and Blackbeard shared some similarities. First is their negative personality. Secondly is Foxy's slow devil fruit is a sub ability of yami devil fruit. Blackbeard's blackhole ability can swallow and disrupt time space that results in destruction and slowdown of time. Since Blackbeard do acquire the quake devil fruit which is a sub ability of yami devil fruit, Foxy's slow power connects well with Blackbeard. I think Luffy defeated Foxy at the Davy Back fight is a parallel to Joyboy defeated the betrayer (assuming he did).
In Chapter 1115, I think the appearance of Demaro Black gave us a hint about the person who betray Joyboy. Blackbeard and Demaro Black share some similarities. First is body shape is belong to the same design category. Second is their negative personality. Third is they have black in their name. Fourth is Demaro Black had a nickname called "Three-Tongued" which mean being good at lying where Blackbeard's jolly roger has three skulls on it.
Demaro Black was known for impersonating other well-known pirates. He took advantage of Strawhatas absence during the time skips by impersonating the Strawhats. During this time, he was smearing Strawhats reputation by doing all kinds of bad things that a pirate will do.
Blackbeard had a few fights, they are nothing special but there are something specifically towards people with D in their name. It's like fate that bind him and other people with D in their name together in such a way that he came out victorious.
Ace - won
Magellan - lost (government pawn, no association with Luffy)
Whitebeard - won (whitebeard was dying)
Dragon - won (revolution moved headquarter)
Boa Hancock - no result (kidnapped Luffy's friend Koby)
Law - won
Garp - won
The giant ancient kingdom with technology so advance and together with Joyboy seems impossible to be defeated by 20 kingdoms. If Betrayer were to steal the technology from ancient kingdom by impersonation, more specifically the ancient weapons. He could have used the ancient weapons on ancient kingdom while the tension of two ideologies are slowly getting higher. This will significantly weaken the ancient kingdom resulting them in defeat.
TLDR:
The person who betrayed Joyboy was probably being a part of the crew and having a negative personality. While journey together, he also taken the smiley symbol as the mark of friendship not to the point of adding into his name. Later an argument separated him from the crew. This led to him held grudge against Joyboy and promise to revenge. He devised a plan to take out Joyboy, the crew, and the persons closely associate with whatever it means by Impersonation, framing, or slandering.
First off, the letter D is a symbol created by Joyboy for his crews. Later they realized they losing the war, so they add the letter D into their name and pass it down generation hoping someone in the future will rise up against the WG. Lily left the crew too early like Vivi did and realized it too late, so she wrote it in a letter to pass it down to her descendant.

There were betrayal in the Joyboy crews, it was hinted by Doflamingo.

I think the appearance of Miss Golden Week in Chapter 1115 kinda hinted that whoever had the yami devil fruit is the betrayer of Joyboy during the void century.
The clues are from her colour traps abilities. Yellow of Laughter indicate Sun God Nika Joyboy where Black of Betrayal indicate the betrayer having yami devil fruit.

The reason of betrayal is because of a simple argument which they are completely opposed of what the other said.
There is a hint from the argument between Shanks and Buggy. Shanks and Buggy have complete opposite personality. Shanks is positive of being brave, selfless, truthful whereas Buggy is negative of being coward, greedy, deceitful.
Buggy hated Shanks so much that make him wanted to kill him.
Since Shanks and Buggy argue about South and North pole, I assumed the argument between Joyboy and betrayer are East and West. If the Earth were rotating from west to east and if one were to travel around the Earth in one circle, in which direction will be faster. This is probably why the red line and grand line exist because red line represents the start and finish line, and grand line represents the race track. whoever reached the finish line first were to be declared winner.

They had a fight and led to one leaving the crew.
Luffy and Usopp are exactly Shanks and Buggy respectively. They share the same personality.

I Think the hint of Foxy appearance in chapter 1115 support my idea of Joyboy defeated the betrayer resulting in leaving the crew. Looking closely Foxy and Blackbeard shared some similarities. First is their negative personality. Secondly is Foxy's slow devil fruit is a sub ability of yami devil fruit. Blackbeard's blackhole ability can swallow and disrupt time space that results in destruction and slowdown of time. Since Blackbeard do acquire the quake devil fruit which is a sub ability of yami devil fruit, Foxy's slow power connects well with Blackbeard. I think Luffy defeated Foxy at the Davy Back fight is a parallel to Joyboy defeated the betrayer (assuming he did).

In Chapter 1115, I think the appearance of Demaro Black gave us a hint about the person who betray Joyboy. Blackbeard and Demaro Black share some similarities. First is body shape is belong to the same design category. Second is their negative personality. Third is they have black in their name. Fourth is Demaro Black had a nickname called "Three-Tongued" which mean being good at lying where Blackbeard's jolly roger has three skulls on it.
Demaro Black was known for impersonating other well-known pirates. He took advantage of Strawhatas absence during the time skips by impersonating the Strawhats. During this time, he was smearing Strawhats reputation by doing all kinds of bad things that a pirate will do.

Blackbeard had a few fights, they are nothing special but there are something specifically towards people with D in their name. It's like fate that bind him and other people with D in their name together in such a way that he came out victorious.
Ace - won
Magellan - lost (government pawn, no association with Luffy)
Whitebeard - won (whitebeard was dying)
Dragon - won (revolution moved headquarter)
Boa Hancock - no result (kidnapped Luffy's friend Koby)
Law - won
Garp - won
The giant ancient kingdom with technology so advance and together with Joyboy seems impossible to be defeated by 20 kingdoms. If Betrayer were to steal the technology from ancient kingdom by impersonation, more specifically the ancient weapons. He could have used the ancient weapons on ancient kingdom while the tension of two ideologies are slowly getting higher. This will significantly weaken the ancient kingdom resulting them in defeat.
TLDR:
The person who betrayed Joyboy was probably being a part of the crew and having a negative personality. While journey together, he also taken the smiley symbol as the mark of friendship not to the point of adding into his name. Later an argument separated him from the crew. This led to him held grudge against Joyboy and promise to revenge. He devised a plan to take out Joyboy, the crew, and the persons closely associate with whatever it means by Impersonation, framing, or slandering.