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. 

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. 

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.