EXCLUSIVE

The Manti Te'o not-really-dead-girlfriend hoax was perpetuated by a man named Ronaiah Tuiasosopo -- who served as a representative for the family of the fake "Lennay Kekua" ... and TMZ has the photo proof.

Here's what we know ...

While Manti was "dating" Lennay ... a Notre Dame die-hard fan who we'll call "Jan" (to protect her identity) reached out to Lennay on Twitter and began an online friendship.

After Lennay's supposed death, Jan became involved with a group called "Wear a Lei 4 Manti" -- in which fans wore Hawaiian leis to ND football games to show their support for the football star.

When the "Wear a Lei 4 Manti" movement began to receive media attention, Jan says she noticed she gained a Twitter follower named U'ilani Rae Kekua.

Jan tells TMZ ... she reached out to U'ilani and asked if she was related to Lennay -- to which U'ilani replied, "Yes, that's my baby sister." The two became friends.

In fact, Jan says they both mentioned how they were going to attend the Notre Dame vs. USC game in L.A. on November 24, 2012 ... so Jan sent U'ilani her cell phone number in case she wanted to meet up.

Here's where the story takes a turn ... Jan tells TMZ she got a phone call from U'ilani on the day of the game directing her to the famous Tommy Trojan statue in front of the stadium, so they could meet up.

But when Jan arrived to the statue, U'ilani was NOT there ... but guess who was -- Ronaiah Tuiasosopo. Jan and Ronaiah even took a photo together (see above).

According to the Deadspin article, Ronaiah was the person who obtained the photo of the woman everyone believed to be Lennay ... and is suspected of having a major role in the hoax.

Jan tells us Ronaiah was with a little girl during the USC meeting who they called Pookah -- and together, they explained how Lennay's sister couldn't come down to visit, so U'ilani sent them instead. Jan says Ronaiah "made it seem as if he was a member of Lennay's family."

After Jan and Ronaiah took the picture together -- Jan says Ronaiah began to act paranoid ... and told her, "Make sure you don't post this photo online."

Soon after the game, Jan says she was contacted by U'ilani ... who also urged her NOT to post the picture of Ronaiah. She didn't explain why.

U'ilani later sent a photo to Jan showing a woman at a cemetery -- and told her, "This is me and my family at Lennay's gravesite. We're spending the day here."

Clearly, the woman in the photo is not U'ilani ('cause she doesn't really exist) ... and some digging suggests the woman in the photo is actually a woman named Donna Tei, who doesn't seem to have a relationship with anyone involved in the hoax.

There's more ... Jan tells TMZ Lennay's "sister" continued to reach out to her after the USC game ... and delivered some more bad news -- that little Pookah had been diagnosed with lupus and was dying in a children's hospital in Orange County, CA.

Jan said she wanted to send flowers -- so Lennay's "sister" gave her an address. TMZ has learned the address belongs to a member of the Tuiasosopo family.

Jan also gave us the phone number Lennay's sister would use to contact her -- and we found out the number is registered to a man named Titus Tuiasosopo -- Ronaiah's father.