William Luca Costa-Marsden Is James Marsden's Youngest Child

James Marsden's son, William Luca Costa-Marsden, was born out of a brief romantic fling. He is one of three siblings, and though his parents separated before his birth, his dad was supportive and more than prepared for his arrival.

William Luca Costa-Marsden is model Rose Costa's only child and "X-Men" star James Marsden's youngest son. The boy recently turned 10, but he already has his own social media accounts.

His Instagram account is currently private and had amassed over 150 followers at the time of publication. Though his posts were unavailable for public viewing, his bio revealed that he currently resides in Los Angeles.

Meanwhile, the bio had the United States and Brazilian flags to reference his roots. William also regularly appeared on his mom's Instagram page, where the model posted sweet tributes for her son and talked about how she parented him.

A little over a week before Valentine's Day in 2022, Rose shared how important it was to her that William learned to be respectful to all people, especially women. The mom of one revealed how her youngster was becoming a well-rounded person who regularly took her out on dates.

William would display all forms of chivalry, opening the door for his mom, pulling her a chair, talking about his day and asking about hers, and even paying for her meal with the money he had earned from doing his chores.

During their mother-son bonding, no electronics were allowed, and afterward, William would even tip the waiters and waitresses. The pair also enjoyed outdoor adventures together, including skiing. According to his mom, the now-fifth-grader is a pro at the winter sport.

William's Father Was Prepared for His Birth

William drew his first breath in Los Angeles on December 14, 2012. It was several months after his parents had separated, but his dad remained in the picture. James supported his son's mother as they prepared for his arrival and even picked a name for the youngster.

The "30 Rock" star, who praised Christina Applegate's strength while she battled multiple sclerosis, was already a father to two children from his previous marriage to Lisa Linde. The union had ended a year before William Luca's birth.

It had been nearly seven years since James cared for a newborn, but he wasn't fazed. In 2014, he reflected on how he felt more than prepared to be a father for the third time, as he told ABC News:

"Once you become a father ... it's like riding a bike – once you learn it, you sort of got it."

His son recently celebrated his tenth birthday, and Rose penned a reminiscent tribute, nostalgically recalling the day William was born. It had been a rainy winter day when Rose "changed completely" and became a mother, as she recounted:

"10 years ago on a rainy winter day you changed me completely, you made me a mother. [...] I saw your little face and I remember having all these emotions - love, joy, happiness and even being scared at same time, but being a a mother to you is the most magical gift that I’ve ever received."

She continued marveling at William's qualities, which included deep empathy and curiosity. The young actress added that she felt magically bonded to her baby. On William Luca's 9th birthday, Rose shared the same sentiments and felt honored to be a mother to "an amazing little boy."

William's Parents Were Together for a Short Period of Time

Per her LinkedIn profile, William Luca's mother is based in West Hollywood and has been signed to Ford Models since October 2006. She also attended Universidade Federal de Minas Gerais, although it's unknown what she studied.

Rose also has a few acting credits under her belt, including in the 2007 comedy-drama "Estomago: A Gastronomic Story" and the 2010 short romantic drama "Luz." The news of her pregnancy with William broke in June 2012.

Rose had had a brief romantic fling with James, whose best movies include 2020's "Sonic the Hedgehog," which resulted in them conceiving William. James had recently divorced his wife of 11 years, but there were differing reports about Rose's relationship status.

One report alleged she had been dating actor Chris Santos when she "hooked up" one night with James. The former couple had been together for two years when Rose relocated from their LA apartment to New York City to further her modeling career.

Chris still wanted to be close to his former beau, so he "won her back" and convinced her to return with him to Los Angeles, and they reportedly spent the holidays together in a new apartment.

Before that return, though, Rose was en route to Brazil during a two-day layover in Miami when she and James bumped into each other and had an intimate night. She confessed to Chris about the supposed infidelity afterward, which left him devastated and led to the end of their relationship.

However, another report claimed Rose and Chris had already ended their relationship before she got involved with James and had met the star before she dated Chris. Since the media speculated in 2012, neither Rose, James, nor Chris have confirmed nor denied the rumors.

William rarely appears on his dad's social media pages, as James is known for protecting his kids from the Hollywood spotlight. However, the occasional posts indicate a strong bond between the two and William's older siblings.

William Has a Bunch of Siblings

William's brother and sister are Jack and Mary. Jack, William's eldest sibling, was born on February 1, 2001. The 21-year-old is a model, signed to Wilhemina Models, and a musician who released his first album, "To The Planes," in 2020.

His older sister, Mary, took her first breath on August 10, 2005. She has appeared on the red carpet alongside her dad and brothers several times, but the young high schooler seemed to live a rather quiet life.