I do feel that there should be a strong case to be made that 2 characters should not be a long enough name to qualify as brandable.
I hope so; would love to hear from you all! If this is legal I am ABSOLUTELY going to release various bots called R2. One of the best things about the R2 name is it feels gender neutral, which is beneficial as a default: (see https://en.unesco.org/EQUALS/voice-assistants )
You might not need a trademark attorney. I have registered several trademarks in Australia using the self-service portal. Obviously every country is different. But the general principles are global. However, the costs can become quite considerable, i.e. many thousands of USD to gain TM registration in most countries.
E.g. there are UK phone brands called “BT”, “O2”, and “Three”.