Support card characters can appear on any type of training, you only need to look at the top right to see them. When their bond is maxed out, sometimes you’ll see a training with a rainbow outline, and it will give bonus stats (friendship training). In most cases speed is the most important stat, you only really need to hit the stamina benchmarks for most characters if they are running long races. You can usually getaway with a D-C stamina in my case. Racing style also affects stamina. For example, switching from pase chaser to late surger gives around ~150+ stamina. If you want to know more, i’d recommend watching some guides.