Anyone can file an Opposition against the grant of a European patent. So if the real protagonist would prefer to remain out of sight, they can instruct someone else to file an Opposition on their behalf. The someone else is commonly known as a 'straw man' or, less commonly, a 'straw person'. The straw person can be a corporate or an individual.
Many of my clients like the idea of concealing their identity in this way. But before pursuing this course, there are important issues to consider.
Perhaps most importantly, there’s always the possibility of a divergence between the interests of the straw person and those of the protagonist. Put another way, the puppet-master must remember that the puppet has a mind of its own.
This is complicated further by the fact that there is an almost universal prohibition on the transfer of Oppositions between parties. (The policy reason behind this is to avoid a market in Oppositions.) Therefore, even if the protagonist would in principle be happy to reveal its identity subsequently in order to take direct control of the Opposition, this is almost always impossible.
So choosing the right straw person is important.
When choosing, it is helpful to remember that the closer the interests of the straw person to the protagonist, the more likely it is that the real identity of the Opponent will be discoverable. However, choosing a straw person with divergent interests, to try to improve the likelihood of continued anonymity, often increases the risk that those interests will diverge further to a point that causes potential difficulty.
Whatever choice you make, no one can guarantee anonymity in perpetuity. Opponents who hide behind a straw person need always to be prepared for the possibility that their identity will be revealed, perhaps through their tactics, though a leak or a hack or in multiple other ways.
So, when considering filing a straw person Opposition, remember that control and anonymity are often in tension. And permanent anonymity can never be guaranteed.
"An opposition is not inadmissible purely because the person named as opponent according to Rule 55(a) EPC is acting on behalf of a third party."