Ok you are talking about Demon Crystals i assume (from thursday's daily)
The platinum archer that is on the trade right now it's Spica, she's a really good unit with high stats
and when awakening comes, she boosts every archer's power
so i highly recomend getting at least one copy; there's also Saki, the black ninja, she's a nice unit to have, she can attack units from afar while been able to be placed on a meelee spot, she's basically an archer in a meelee spot. In my opinion Spica > Saki
The other archer, Bashira, is no longer there, she was only for a certian amount of time, now can only get her by gacha (i believe). Her skill is triple shots, making her a 2nd hand Nanaly (5 shots) but still awesome
Take into mind that the trading post changes from time to time, rotating beetween a pool of certain units, so if you missed one they'll be back just need a little patience. Since the game first came out and present time, it's only changed one time (2 sets of units) so it might take a while for it to come all the way back to the unit you want/need
Also very important, never buy fairies from the trading post, it's a scamm
until awakening fairies come
so just focus on units you like/think might be usefull for you