WHEN OLIVIER Widmaier Picasso began researching his family history to bring his book Picasso: An Intimate Portrait to life, he estimates he knew 30 percent of the story. “Now maybe I’m at 85%,” he says. “I still have things to learn.” Widmaier Picasso’s biography of his famous late grandfather, whom he never met, traces Pablo Picasso’s complex family tree, his relationships with women, and his involvement in political activities to understand how each influenced his work and the man he was. “It was like trying to open a door locked for years and bringing the past to life for the first time,” Widmaier Picasso says.

Widmaier Picasso, who studied law but now works as a television producer and consultant, tried to bring a lawerly, unemotional approach to his research. “Different members of my family didn’t share the same vision [of past events],” he says, “so I needed to understand why one person was telling me this and another person was telling that.” Though Widmaier Picasso’s grandmother, Marie-Thérèse Walker, never married the artist, they were together for 16 years. Widmaier Picasso writes of their initial meeting in 1927 outside the Galeries Lafayette in Paris, “She had not the slightest idea who this Picasso might be, but she noticed his superb red and black tie—which she would afterwards keep all her life.” After they separated, Picasso remained a critical part of Marie-Thérèse’s life. The two stayed in touch, speaking regularly on the phone, until his death in 1973, and he even sent her an allowance. Four years later, she committed suicide.

While they were together, Marie-Thérèse was the artist’s muse, and one of Widmaier Picasso’s favorite of his grandfather’s works is a portrait of his grandmother in a bright red beret, set against a crimson background. He spoke to WSJ. about it and four other of the Picasso paintings he loves most.

1. Yo Picasso, Self-portrait, 1901

“This painting is a masterpiece that reveals Pablo was arriving in Paris with a firm intention to succeed in painting. There is pride and talent in the execution. He is 19 years old, and he has already had some success in Barcelona. So for me, it’s the beginning, when Pablo becomes Picasso. It’s a symbol of the power of youth. It’s in 1901, the beginning of a new century, and he wants to be part of it.”

2. Still Life with Chair-Caning, 1912

“When you see this work, you understand that it’s a coffee table, in a bar anywhere in Paris. It reflects the ambiance of Paris at the same time it reflects Pablo’s innovation. It’s the second part of his cubism period, known as ‘synthetic’ cubism [when he started to incorporate letters and figures into his work]. He’s not beholden anymore to what he sees, he’s painting what he knows. The painting is a symbol of his creativity and unique style.”

3. Paul Drawing, 1923

“Paul, my uncle, died in 1975, just two years after Picasso. As Pablo’s first son, he has been accused of being a nobody, of being someone with no talent or education. In fact, he was more important than just being a son of Pablo. What is important to me is that Pablo kept this painting during his whole life. It shows the love of a father for his children, and it also shows the life with [his first wife] Olga and with their son, and the happiness they had.”

4. Portrait of Marie-Thérèse in a Red Beret, 1937

“My grandmother was not a traditional grandmother. I remember she was very loud: speaking loudly, laughing very loudly. Pablo made her free. She was the only one whom he gave a huge number of artworks, so she never had to worry about life. After Pablo, there was no other man. I finally understood that she killed herself four years after his death because she knew after the death of Pablo, she would be unable to lead a normal life. When I picked up this painting, I thought the look of Marie-Thérèse was very modern. This painting makes me feel happy.”

5. The Musketeer, 1969

“In 1969, Pablo was 88 years old and he was still painting in large format. This size is very typical of contemporary art. I think that he was pioneering in this kind of expression. I see a link between him and Jean-Michel Basquiat a few years later. But at the same time, it’s a musketeer, a classical figure. It’s referring to the influence of Old Masters on Pablo. In the 1950s, he was painting after Delacroix and Monet, reinterpreting the great paintings of the past.”