Adam Back is a British cryptographer and cypherpunk born in 1970. He has earned a PhD in computer science from the University of Exeter. Adam Back is most known for his invention of the proof-of-work system, which he originally adopted in his anti spam system called hashcash.
Adam Back has never contributed directly to the Bitcoin source code, but his ideas helped shape Bitcoin. Bitcoin was influenced by hashcash and the proof-of-work system it uses. Hashcash and Adam Back are both quoted in the Bitcoin whitepaper. Bitcoin uses the proof-of-work system to avoid the double spends of transactions; Without proof-of-work Bitcoin could not exist the way it does today.
Adam Back is currently the CEO of Blockstream, a for-profit Bitcoin development company that employes a few Bitcoin Core Developers. Through this company developers are paid to work on the Bitcoin source code and improve the protocol as a whole. Blockstream and the developers funded by the company are currently responsible for less than 20% of the changes that happen to the Bitcoin source code.
Even though the amount of changes done to the Bitcoin source code by Blockstream are less than 20%, some people worry that Blockstream might gain too much power and be able to influence the Bitcoin Protocol.
This is not much of a real concern because if people do not approve of the changes done on the Bitcoin source code, they can just not implement them, and run a different version of the source code.