William was seen giggling as his middle child stared down at the crowds of press, while Kate Middleton held her newborn son in her arms.
It wasn't the first time Charlotte showed her cheeky side either after being spotted at Prince Harry's wedding in May poking out her tongue as she arrived for the service. Van Cutsem is one of four brothers who have been close to William and Harry since they were little.
The Cambridges have previously chosen close friends and confidantes for the role.
Mr. Harry Aubrey-Fletcher attended both Ludgrove prep school and Eton with William.
In addition to similar facial expressions, Princess Charlotte also appears to share similar lips, eyes, and nose as her mom.
Kate, dressed in an ivory dress and matching hat, and William were joined by William's father Prince Charles and his wife Camilla, along with younger brother Prince Harry and his wife Meghan, dressed in olive green.
Proud grandfather the Prince of Wales and Duchess of Cornwall did attend, along with the Duke and Duchess of Sussex who walked hand-in-hand.
The gown Louis worn was created in 2008 by fashion designer and dressmaker Angela Kelly.
The Choir of Her Majesty's Chapel Royal performed This is the day which the Lord hath made, which was composed for The Duke and Duchess of Cambridge's wedding in 2011, and Suo Gan, a traditional Welsh lullaby.
The little prince was also christened in the Lily Font, a silver gilt font and bowl decorated in lilies, ivy and cherubs.