Robert "Bobby" Costello started as the U.S. Eventing Team Chef d'Equipe and Technical Advisor for the program in 2022. With a decorated career as an international athlete, actively competing up until 2010, Costello was a member of the World Championship Team and placed in the top 10 as an individual at the Sydney Olympic Games in 2000. As the U.S. Technical Advisor, Costello led the U.S. Eventing Team to several key championship results, including a team silver at the 2022 FEI World Eventing Championships in Pratoni del Vivaro Italy and a team silver at the 2023 Pan American Games in Santiago, Chile.  

James Alliston is a dedicated eventing athlete and trainer, known for his commitment to developing both horses and riders. He grew up in Gloucestershire, England, where he was involved in Pony Club, honing his skills in various disciplines. After taking a gap year from university to immerse himself in the horse industry, Alliston apprenticed for U.S. Eventing athlete Bruce Davidson, Sr. This experience influenced his training philosophy of emphasizing the importance of producing fit, competitive horses. 
 
In early 2022, Alliston changed his nationality from British to American after residing in the U.S. for over 15 years. Soon after, he was selected to represent the U.S. Eventing Team at the FEI Eventing Nations Cup in Boekelo, Netherlands. He and Alliston Equestrian’s 8-year-old gelding, Nemesis, served as pathfinders for the team. 
 

Over the years Alliston has continued to achieve success at the CCI4* and CCI5* levels. In 2023, he once more represented the U.S. Eventing Team at Boekelo, this time riding Ric Plummer and Alliston Equestrian’s mare, Karma, where they finished on their dressage score of 35.9, leading the U.S. Eventing Team to a fourth-place finish. Alliston and Karma were named to the Defender U.S. Eventing Team for CHIO Aachen in 2024. The pair not only produced a double clear show jumping round, but assumed the role as pathfinders and crossed through the timers with just 2.4 time penalties to finalize their outing on a score of 38.4, finishing ninth overall individually and as the highest-placed team combination in the 2024 SAP-Cup CCIO4*-S. 
 
James owns and operates Alliston Equestrian and East Bay Riding Academy in Castro Valley, Calif., where he trains alongside his wife Helen. 

In 2018, Coon expanded her training by working with Richard Waygood and Olivia Oakley, supported by the Karen Stives Endowment Fund Grant from the United States Equestrian Team (USET). In 2019, she was named to the USEF Developing Potential Training List, marking her first inclusion on a high-performance roster.  
 
In 2024, Coon competed as a member of the Defender U.S. Eventing Team at the FEI Eventing Nations Cup in the Netherlands, where the team finished in second place. Coon and her horse, Cute Girl, achieved notable success, securing third place individually—marking one of the highest placements for a U.S. combination in nearly a decade. 

Coon participated in various competitions throughout 2024, including events in Le Lion d'Angers, Boekelo, and Aachen, where she consistently demonstrated strong performances on multiple horses. 

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. 

Phillips began her equestrian pursuits at a young age, initially riding Western before transitioning to English riding and eventually eventing at around six years old. Drawn to the thrill of cross-country and jumping, Alyssa pursued her passion for horses throughout her childhood and into college. 
 
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. In 2017, Alyssa began training with Jennie Brannigan, gaining valuable experience in the competitive world of eventing. 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 established her own operation in Ocala, Florida, where she manages her horses and staff, finding the experience both challenging and rewarding. 
 
Phillips was selected as the traveling reserve for the Pan American Games in 2023, an opportunity that provided significant learning experiences despite facing setbacks that year. She participated in the development tour, achieving noteworthy results, including a second-place finish at Millstreet. Career highlights include winning the 2021 Land Rover Kentucky Three Day Event CCI4*-S, winning the 2023 Rebecca Farm CCI3*-S, and winning the 2024 Event at TerraNova CCI4*-S with her horse Oskar.