A 24-year-old woman might have gone missing because she wanted to avoid further treatment for her eating disorder, says her brother.
Alice Vulliamy, from Llangynidr, near Crickhowell, Powys, vanished on 6 February after her car was found locked near Christ College, Brecon.
Andrew Vulliamy said his sister had been under pressure to "increase her food intake".
He appealed to people with information about his sister to contact the police.
The search has centred on Brecon and Llangynidr, while kayaks have been used along the River Usk.
Dyfed-Powys Police are continuing to look for her, and mountain rescue and dog teams are said to have put in 1,500 man hours in the hunt for her.
But police said last week they were becoming increasingly concerned for her welfare.
The last reported sightings of Ms Vulliamy were made in Llangynidr on 6 February, while an unconfirmed sighting was made in the village on the following Monday.
Mr Vulliamy said: "I am pleading for people who may have seen Alice to call police urgently.
"Secondly, I have a message for Alice herself. We'd like her to let us know she is safe, and for her to know that no-one is angry or upset with her."
The last sighting of his sister was near her family home in Llangynidr.
'Confident and talkative'
He said that she was "very confident and talkative", but was probably anxious and frightened.
"Even though she is 24 she could be mistaken for a teenager," he added. "The motivation for her disappearance might be to avoid the treatment she was receiving for an eating disorder.
"Alice had been under pressure to increase her food intake."
But Mr Vulliamy said the police remained positive and were investigating the possibility she had sought refuge with a friend or someone she knew.
Mark Jones of Brecon Mountain Rescue said his team had been involved in the search from the start.
The other teams in the area, Longtown from Abergavenny, Western Beacons from Bridgend, and Central Beacons from Merthyr Tydfil have assisted in the search.
Mr Jones said the Brecon team used kayaks last week to search along the River Usk between Brecon and Llangynidr, and the four mountain rescue teams, along with dog search teams, had put in about 1,500 man hours in the hunt for Ms Vulliamy.
Ms Vulliamy is white, 5ft 3in, with mousy brown, shoulder length hair. She is of slim build and also wears glasses, but they were found in her car.
People with information can contact Dyfed-Powys Police on 101.