Who are they? Would be interested in any kind of information.
most of the time people use the word "market maker" to create an imaginary entity for themselves to justify the unexpected incidents with price. (at least that is my opinion).
for most altcoins the market maker is simply the creator (developers) of that altcoin. because most of these altcoins we see on coinmarketcap from first altcoin to the last have some kind of premine, ICO , etc to make the owner as a whale without any trying. (like the big ETH ICO and Dash premine or Ripple for that matter).
also in smaller altcoins you can become the market maker if you know how the market works and have enough money and influence to have some effect on it.
for example a couple of days ago, during the litecoin pump, a simple Signaling of SegWit by F2Pool could cause price rise and another simple tweet of him caused a drop and the third tweet picked the price up.