I suggest an attacker at the start, maybe Zhuge Liang (for the unique) or Jeanne d'Arc (swimsuit) or Ba'al (but her unique works only if her hp are over of 50%). Alone, they work good unlike other cards.
Zhuge Liang I wouldn't recommend, honestly, for a first plat compared to other options available. Neither would I suggest SS Jeanne, since her Unique randomly chooses a unit in your deck to have guaranteed crits, and for newer players their deck probably doesn't have strong units to make use of the crits.
Ba'al is definitely a great first plat. Earth decks are the cheapest deck to build, and is thus very beginner-friendly.
It's not up-to-date, and OG Jeanne is outdated now so I wouldn't advice getting her unless they buff her in the future. Other notable plats are Remix Asmodeus and Valentines Guna, though building a deck around them are very expensive.