Under Congolese law, anyone under 18 is considered a minor.
The Romanian military observer is among five peacekeepers accused of acts of sexual abuse and exploitation in the first three months of 2017, a senior UN official told the BBC.
UN peacekeeping missions have been hit by a raft of child sex abuse scandals.
The underage girl involved in the first case has been put under the care of the UN children's agency Unicef, Adama Ndao, head of the conduct and discipline team for the UN peacekeeping mission in the country (Monusco) told the BBC.
All the other cases involve adults.
Two of the complaints come from women who are demanding paternal recognition from the peacekeepers they had sex with, Ms Ndao added.
She said that one of the women has already had her baby, while the other is still pregnant.
Two South African soldiers and two non-military Monusco officials, from Burundi and Niger, are among the accused.
All have been suspended pending the outcome of investigations.
Other complaints about UN peacekeepers which have been received and are under consideration, implicate police officers and soldiers from Senegal, Uruguay and South Africa, Monusco spokesman Charles Bambara said on Wednesday.
UN and French forces faced multiple allegations of child rape in the Central African Republic, a scandal which was compounded by allegations of a high-level cover-up.
A UN peacekeeper in the Democratic Republic of Congo has been suspended over allegations that he fathered a child with an underage girl.