Many royals never reigned but were recognized nominally and acknowledged as kings :
If you mentione Louis XVII, then Napoleon II and IV come to mind.
Also Louis XIX, Henri V, Jacques I, Alphonse II or the present Louis XX (Alfonso).
Philippe VII (Count of Paris), Philippe VIII, Jean III, Henri VI, Henri VII and Jean IV.
Portuguese monarchics count D.Miguel II, D.Duarte II and D.Duarte III.
Spanish Carlists count Carlos V, Carlos VI, Juan III, Carlos VII, Jaime I, Alfonso Carlos I and eventually Javier I, Carlos VIII (Hugo).
Not sure if his son calls himself Carlos IX.
The late Count of Barcelona is referred to as Juan III by many spaniards
The Stuart claimants maintained their numeral sequence, James III/VIII, Charles III and Henry IX/I, not to mentione the rest of the line up to the current king Francis I aka Duke Franz of Bavaria.
You can add King Tomislav II of Croatia (Duke of Aosta), King Fredrik Kaarle of Finland (Dk.Hesse) or king Mindaugas II of Lithuania (Dk.Urach).
The Emperores of Austria and the kings of Italy and Spain among their titles claim the nominal title of king of Jerusalem.
A sovereign that never reigned due to mental haqndicap was Ludwig II of Baden.
He succeeded his father Leopold I in 1852 but his next brother Friederich acted as regent during his reign.
Eventually in 1856, two years before his death, his brother deposed him and took the throne.
King D.Afonso VI of Portugal started his reign under his mother's regency, also due to mental illness.
After a palatial coup, he took the reins of power (nominally) before his brother D.Pedro (II) overthrew him again and imprisonned him.
D.Pedro acted as regent during his brother's lifetime seizing the crown only after his death.
Say what you want about the Portuguese crown prince who was assassinated alongside his father in 1910; but Louis XVII (only son of King Louis XVI and Queen Antoinette of France) has been officially acknowledged in history as King of France -- but only in name. If nothing else, his postnominal number 17 is respected.
Anyhow, I was wondering about other examples of royal men who never really reigned, insofar as they never exercised the constitutional functions and powers of sovereigns, but who have nonetheless been recognized at least nominally as kings.
I've already mentioned King Otto of Bavaria, who was already declared mentally incapacitated when in 1886 his older brother (King Ludwig II) died under mysterious circumstances. So his entire reign was under a regency. Can anybody name other examples of adult monarchs who never exercised any sovereign powers during their reigns? I believe some were deposed later on for being mentally unfit (e.g. Emperor Ferdinand of Austria), or had regencies declared (e.g. King George III of Great Britain). But they don't really count, since they presumably were mentally competent earlier on in their lives and reigns.
Now: some boy kings might count, if they never obtained their legal majority -- defined differently in different countries. Simeon of the Bulgarians, for instance, reigned for only three years, from the age of 6 to 9; so he might count. But King Peter II of Yugoslavia eventually obtained his legal majority after coming to the throne as a boy, and ruled briefly getting deposed. I don't know if King Edward VI of England would count, since he died at the age of 16 -- whether he ever exercised any real power (it was a day when monarchs ruled, not just reigned).
Can anybody point out other examples of monarchs who reigned in name only, whether because of age or disability -- or other circumstances (e.g. politics, revolution)?