What word describes someone I know exists but have never met?
Solution 1:
I am looking for a word to describe someone I know exists but that I have not yet met in person or virtually.
I suggest a checkbox labeled "Know of" for people you know of, but do not know personally.
know of (phrasal verb transitive)
(Know of someone/something) — to know that someone or something exists and who, what, or where they are Macmillian
Perhaps you should work backwards and think first of the uses you could have for this category or categories (e.g. sending holiday greetings, following up for more information, adding to a family mailing list for newsletters, emails, etc.) and then deciding on a checkbox label for each purposes.
Solution 2:
In an app, it's best that the word or phrasing is unambiguous, especially if users include those who are not fluent English speakers.
I know Mary "only by name". Alternative versions include: I know Mary by name only. I only know Mary by name. Mary is someone I know only by name. etc.
"Only" is mostly an emphasis to clarify the familiarity, and may be omitted in some context. -Do you know any actor? -I know Kevin Bacon by name. Here, "by name" gives just as much information as "by name only", because it's presumed the speaker does not know Kevin Bacon personally.
"By name" indicates the least amount of past interaction among the suggested words.
A closely related verb form is "hear of", as in "I have heard of Mary". It is similar to "know of" but "hear of" indicates that the two parties never met. Furthermore, "hear of" may or may not include knowing anecdotes of the person, but "by name" is essentially only knowing the name.
Solution 3:
Best expression I can think of for that is "friend of a friend". Obviously this isn't literally true in all cases, but offering "mother of a friend", "second cousin once removed of a friend", etc. would be rather more options than is reasonable.
According to Is there a single word for a "friend of a friend"?, the most likely single word is "acquaintance", but I agree with your reasons for ruling that out.
Solution 4:
If I were a politician I might call such a person a:
“known unknown”
but I’m not, so I think it might be more appropriate to reverse this as an:
“unknown known”
Coda
I originally wrote “You read it here first.”, but there is apparently a movie of that name, and it appears that after the original usage all the known and unknown combinations have been explored. Still, in this context, I think it‘s rather good. (Of course it’s two words, but if there were a single word that fitted, it would have been suggested already. You can’t have jam on it as well.)
Solution 5:
It seems you are looking for a category of “contacts” or “connections” (people) whom you don’t personally know.
Since you mentioned Kevin Bacon, I thought of six degrees of separation:
. . . the idea that all people are six or fewer social connections away from each other. As a result, a chain of “friend of a friend” statements can be made to connect any two people in a maximum of six steps. . . . The idea is sometimes generalized to the average social distance being logarithmic in the size of the population.
LinkedIn, for one, labels people as 1st-degree, 2nd-degree, or 3rd-degree connections relative to you.
1st-degree is someone you know. 2nd-degree is someone connected to your 1st-degree. 3rd-degree is someone connected to your 2nd-degree.
Working with that, perhaps you could imagine the structure as:
Contact
Degree
Where your friend Bob is a Contact and his mother Mary is a Degree
degree noun
1 : a step or stage in a process, course, or order of classification
// advanced by degrees
// We all know that you’re only three degrees away from all sorts of interesting and even famous people on social media.— Alex Proud
. . .
3 genealogy : a step in a direct line of descent or in the line of ascent to a common ancestorSource: Merriam-Webster — degree
I’ve included the definition at 3 to suggest that you could use the term more figuratively — beyond just the literal step — to suggest a relationship (even if it’s not a familial one).