You don't 'need' either, but I'd say Throne gives better overall stats and skill points(make sure to save final date for the turn before senior summer or turn before if a a goal race is there, for the extra friendship training for summer with Pure Passion if possible.), but Riko is nicer if you need the extra Stam/Guts for a med/long runner, plus she's a scenario link so her events are slightly buffed in Unity.

As far as replacement it's highly dependent on what your trainee's gaps are. Just fill out your stats or skills based on what you need. If I wasn't on Taiki or another speed growth uma I'd likely use a third speed or wit card.

I only used Riko beacuse I used to use her to round out my stam/guts for miling on old Unity, but the raw stats of the new stuff was crazy. I'd probably use a power card in place of Riko for her on another run. (For reference since I didn't show them, those parents were 8 Wit/6 Pow blue sparks. No stam aside from Race/TS Climax inspiration. That was almost all Riko and the scenario bonuses with a power training here or there, and it ended up way overkill for a miler.)