Mbappe should be regarded as a PSG legend, having spent seven successful seasons there and ultimately leaving as their all-time leading scorer with 256 goals in 308 matches.
However, his legacy was somewhat marred by the way he left, with many feeling that during the latter half of his time in Paris, he was merely biding his time for the right opportunity to join Madrid, the club he had long aspired to play for as a child.
PSG, under their Qatari president Nasser al-Khelaifi, were displeased with how Mbappe chose to let his contract run down in order to sign for Real in 2024, which deprived them of a transfer fee.
A contentious legal dispute has persisted between the parties for much of the time since, with Mbappe asserting that he is owed 55 million euros ($64.4 million) in unpaid wages and bonuses from his time in Paris.
The latest development occurred just this week, when one of Mbappe's attorneys informed AFP that the France captain had retracted a complaint of moral harassment against his former club.
This followed the Paris prosecutor's office revealing last month that an investigation had been initiated after a complaint from the player regarding how he was treated by PSG in the summer of 2023.
He believes he was marginalized by PSG and forced to train with players the club intended to offload after he declined to agree to a new contract.
Mbappe missed a pre-season tour to Japan and the beginning of the next season before eventually being reintegrated into Luis Enrique's squad.
All of this should have been behind Mbappe long ago, considering how his first season at Real has unfolded on a personal level.
The 26-year-old, who won the World Cup in 2018, scored 43 goals in 56 matches for his new club across all competitions by the end of the La Liga season, an impressive tally.
However, Mbappe has faced challenges at the Club World Cup, not participating at all during the group stage due to a stomach illness that required hospital treatment.
First start?
In his absence, young forward Gonzalo Garcia has stepped up remarkably, starting all five matches in the United States and scoring four goals.
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The last of those was the opening goal in the 3-2 quarter-final victory over Borussia Dortmund at the MetLife Stadium on Saturday, but it was Mbappe who ultimately scored what proved to be the decisive goal.
He came off the bench midway through the second half and netted a stunning, acrobatic overhead kick for Real's third goal of the day in stoppage time.
"He is still not at his best, not 100 percent, but he is improving every day," Alonso commented on Mbappe after that match.
“Now he will have three days to continue progressing and feeling better before the semi-final.”
It is difficult to envision Mbappe not making his first start of the tournament against PSG, the club that won the Champions League in the season following his departure after years of disappointment in Europe while he was part of the team.
PSG arrived in the US fresh off a 5-0 thrashing of Inter Milan in the Champions League final.
They reached the semi-finals with a 2-0 victory over Bayern Munich in Atlanta in the last eight - despite having Willian Pacho and Lucas Hernandez sent off - and need not fear Real.
"It doesn't matter who we face in the semi-finals. What matters is that we are there and that we want to reach the final,” said Luis Enrique, for whom this is also a significant occasion given that he spent five years at Madrid as a player in the 1990s.
Alonso has recently taken over as Real's coach after an impressive tenure with Bayer Leverkusen and has already shown great tactical adaptability, switching between a back four and a three-man central defense during the tournament.
It will be intriguing to see which system he chooses here, and if Mbappe starts as he prepares to face PSG for the first time since July 2017, when he was still an exciting teenager at Monaco.