Robert "Bobby" Costello started riding in as a child in Massachusetts, competing at local shows and fox-hunting with the Myopia Hounds. He found the sport of eventing and worked his way through the ranks as a professional, ultimately competing at the highest levels in the sport of eventing.

Costello became an upper-level staple for the program, representing the U.S. Eventing Team at the 2000 Sydney Olympic Games in Sydney Australia, taking eighth individually with Chevalier. The pair also secured top results at Bughley CCI5*-L and the Rolex Kentucky Three-Day Event. Costello finished seventh individually at the 2003 Pan American Games with Dalliance, before turning towards coaching and eventually stepping away from riding full-time in 2010 and focusing solely on coaching and training. 

Since he joined the program in 2022, Costello has helped several senior teams produce significant results at major international competitions and championships including a team silver at the 2022 FEI Eventing World Championships in Pratoni del Vivaro, Italy, and a team silver at the 2023 Pan American Games in Santiago, Chile. Under Costello's tutelage, the team has also secured two-consecutive second-place finishes in the CCIO4*-S at CHIO Aachen in 2023 and 2024. 

California-born Elisabeth “Liz” Halliday began riding when she was 8 years as an active member the United States Pony Club. Halliday moved to England in 2000 to work for famed eventer William Fox-Pitt and was based in Europe for many years before bringing her operation to the U.S. with locations in Ocala, Florida, and Lexington, Kentucky, in 2020. That same year, she earned the distinction of USEA Rider of the Year after racking up nine wins and 23 total top-ten finishes at FEI competitions over the season.

Halliday achieved her first CCI5*-L completion in 2016 at the Kentucky Three-Day Event with Deborah Halliday's HHS Cooley. She has also brought numerous horses to top-ten finishes at the level, including Ocala Horse Properties, LLC's Deniro Z, The Monster Partnership's Cooley Quicksilver, and The Nutcracker Syndicate's Cooley Nutrcracker.

In 2023, she brought Deborah Palmer's and Ocala Horse Properties, LLC's Miks Master C to his CCI5*-L debut at the Kentucky Three-Day Event where they finished in third-place overall and were reserve champions in the USEF CCI5*-L Eventing National Championship. In 2023, the pair were the Adequan USEA Advanced National Champions at the American Eventing Championships.

Halliday has represented the U.S. Eventing Team in several FEI Nations Cup competitions, including the 2022 CCIO4*-S at Bromont in Canada, where she rode Miks Master C to individual victory and led the U.S. team to secure the win. In 2023, the pair were part of the silver medal-winning team at the Pan American Games in Santiago, Chile.

Halliday continued her consistency in 2024, marking several top results with both Cooley Nutcracker and Miks Master C, with a top-ten finish coming for Cooley Nutcracker at the Kentucky Three-Day Event CCI5*-L in April. The pair were called onto the U.S. Olympic Eventing Team for the Paris 2024 Olympic Games where they helped the team finish sixth overall and 19th individually. 

In addition to her equestrian pursuits, Halliday was a professional driver in sportscar and GT endurance disciplines, competing from 1997 to 2010 before focusing on her equestrian career. 

Boyd Martin, the son of Olympic speed skater Toy (Dorgan) Martin and Olympic cross-country skier Ross Martin, began riding at a young age in his native country of Australia. He made his debut at the CCI5*-L with a win at the 2003 Australian International Three-Day Event riding the Thoroughbred gelding True Blue Toozac.

He moved to the U.S. in 2007 to further his career as an eventing professional, serving as the assistant trainer to Olympian Phillip Dutton for two years before starting his own business. Martin began riding for the U.S. in 2009.

Martin is a three-time Olympian, having represented the U.S. in London 2012, Rio de Janeiro 2016, and Tokyo 2020. He is a four-time U.S. Eventing Team member for the FEI Eventing World Championship, including as a part of the silver medal-winning team in 2022 in Pratoni, Italy. With Pancho Villa, he was a member of the U.S. team that won gold at the 2015 Pan American Games, and in 2019, he won team and individual gold at the Pan American Games with Christine, Thomas, and Tommie Turner's Tsetserleg TSF.

Twice, Martin has been the USEF CCI5*-L Eventing National Champion at the Kentucky Three-Day Event. First in 2019 with Tsetserleg TSF and again in 2021 with Christine, Thomas, Tommie, and his own, On Cue. In October 2021, he won the inaugural Maryland 5 Star with On Cue, becoming the first American to win a CCI5* since 2008.

In 2023, Martin piloted his string to several significant top finishes at the CCI5*-L with five top-ten finishes at events around the world, including two top-ten finishes for Fedarman B at both Luhmuhlen and Pau. Martin brought both Fedarman B and Commando 3 forward as potential Olympic selections at the start of the 2024 season. A strong showing of both at the CCI4*-S at the 2024 Kentucky Three-Day Event did not make selection easy, but Martin ultimately chose Fedarman B as his mount of the 2024 Olympic Games, with Commando 3 as a direct reserve. Paris 2024 will be Martin's fourth consecutive Olympic appearance for the U.S. Olympic Eventing Team. 

Martin and Fedarman B were successful in Paris, adding just 1.6 to their initial dressage score of 30.5. The duo were one of only seven combinations to jump clear in both rounds on the final day, finishing tenth individually and contributing to the team's sixth place. 

Martin and his wife, professional dressage trainer and athlete, Silva Martin, own and operate Windurra USA in Pennsylvania.

Alyssa Phillips had a successful young rider career, winning individual gold at the 2011 Adequan®/FEI North American Youth Championships presented by Gotham North, team gold at the NAYC in 2011 and 2012, and team silver in 2014. With Bliss III, Phillips had top-10 finishes in CCI4*-S events at Rebecca Farm in 2015 and Richland Park in 2016. With her Holsteiner gelding, Oskar, she won the CCI3*-L U25 at Bromont in 2019 and had top ten finishes in CCI3*-S events at Carolina International and Plantation Fields and in the CCI4*-S at Ocala Jockey Club. Phillips has been a member of the USEF Eventing High Performance Programs for several years and has represented the Development Team in international competition. 

Phillips has represented the Defender U.S. Eventing Team on several occasions as she becomes a mainstay for the program at the upper levels. She helped the team finish fifth in the FEI Eventing Nations Cup Netherlands CCIO4*L in 2022, and was a member of the team at the 2024 CHIO Aachen, where they captured their second consecutive podium finish and finished 14th individually.