Every thousand years or so a Kwami can grant their holder eternal life, but it comes with a 'price' if the kwami grants their holder this, said holder will gain knowledge of the entire universe AND gain a part of the kwami's soul.

Tikki granted Marinette this gift, after she defeated Hawkmoth, because Marinette had gone through a lot more than all of Tikki's previous holders. With the whole, being adopted, and then Hawkmoth came around, and then Lila. . . . Tikki gave the girl this last gift before the miraculous were destroyed. and Marinette traveled the world, sure, she still aged, but every 10 or so years would only age her 1 year.

On one trip through Gotham, Marinette met another of the blessed of the Kwamis, a dude that had been blessed of Dussu, named Alfred. They fell in love, and when the people Alfred worked for died, Marinette helped Alfred raise Bruce, and once Bruce was 29-ish, Marinette left again, promising to return.

Marinette was back in Gotham, strolling in Gotham Park when she recognized someone, well, actually two people.

The Wayne's were used to people walking up to them, but when this woman walked up to them, they were completely unprepared, she walked right past the boys, and stopped at Bruce, tilting her head, she said

"Awwww. Bruice-bear you've grown so much!!" then she turned to Alfred and gave him a hug. "Alfred my dear. How long had it been?" the batboys were just staring at them jaws on the ground, and then, Dick noticed something

"Alfred, are you blushing?" Alfred was in fact blushing. Bruce smiled, and pulled the older lady into a hug. "Marinette! Where've you been??" The old lady, now dubbed Marinette smiled at him.

"I went everywhere. Every continent, every island, every country, all the states and provinces, all the major cities, and some of the minor ones. Honey, I even went to Antarctica."

"Honey??!!?" Dick asked incredulously, and Marinette turned to look at him.

"And who might you darling?"

Dick flushed at the sudden attention

"Dick Grayson-Wayne"

Marinette gasped. Then proceeded to turn and glare at Bruce.

"BRUCE WAYNE! YOU DID NOT ADOPT A HOARD OF CHILDREN!"

Alfred laughed lightly at Marinette's exclamation, Marinette whirled on him, shaking her finger at him.

"Don't you dare laugh! Just because you're the love of my life DOESNT MEAN YOU GET AWAY WITH THE FACT THAT YOU LET BRUCE ADOPT A SMALL ARMY!!"

"I'm the love of your life?" Alfred asked softly, knowing for an immortal, that was a large statement.

Marinette's face softened into a smile.

"Yes dear. You were never just a passing lover to me"

"Auntie Mari? You and Alfred... you know Alfred and Father?" Marinette turned again, this time to face her beloved god-nephew Damian

"Dami dear! Didn't I ever tel you about my love and the child we raised? This is them. I suppose that means that you know who your father is now?"

Damian nodded, a look of understanding flashed across his face.

"Back the hell up." Jason half-growled. "WHO the actual fuck are you?"

Marinette grinned at him.

"Marinette Pennyworth at your service. You might know me as MDC though. I helped raised Damian for a bit, and I raised Bruce"

"You're married yo Alfred?!" Dick exclaimed at the same time that Tim gasped

"You're MDC?! They've been rumored to be immortal! I can't believe I'm in the presence of THE MDC!"

Marinette laughed.

"Alfred and I are currently common-law married... but" turning and directing her next statement at Alfred, she continued "If he'll have me... I'd like to marry him"