Ilsa is one of Strike’s most longtime friends. They attended the same primary school when he was living in St. Mawes. She is described as “fair-haired and bespectacled.” Ilsa’s mother is friends with Strike’s Aunt Joan. At Cormoran and Nick’s joint eighteenth birthday party in London, Ilsa met Strike’s friend Nick. They dated for a year but broke up when each went to separate universities. They then got back together years later in their midtwenties and were married. Strike was the best man in their wedding. It’s stated that Nick and Ilsa are “the only place where where the two halves of Cormoran’s life intersected: London and Cornwall, happily married.” Ilsa now works in London as a lawyer and lives on Octavia Street in Wandsworth with Nick.
Ilsa is a very caring friend, worrying (and rightfully so) in The Cuckoo’s Calling that Cormoran has gone underground because of “woman trouble.” She’s very perceptive, picking up on Strike’s attraction toward Robin, whom Ilsa likes. In fact, at one point, Ilsa looks “slightly aggrieved at the news that Robin still intended to marry someone other than Strike.”
In the second book, The Silkworm, Strike calls Ilsa on behalf of his client, Leonora Quine, who needs legal help. Nick and Ilsa also attend Strike’s birthday dinner hosted by Strike’s sister Lucy, at her house. Nick and Ilsa give Strike the gift of a bottle of Arran Single Malt whisky, his favourite, but it reminds him of Charlotte, with whom he first tried it. Later in the book, Strike stays at Nick and Ilsa’s house to avoid the press “crawling all over Denmark Street.”
Later, at a party hosted by Nick and Ilsa, Strike meets Elin, a girlfriend he has throughout Career of Evil.
Also in Career of Evil, Strike stays at Nick and Ilsa’s house to avoid the press on Denmark Street. They are very welcoming to him, telling him that he can stay as long as he wants — Ilsa even comments “on the fact that Strike moves around the house without leaving any real mark on it,” due to his tidiness. Staying in their white, austere guest room, Strike is painfully aware of the fact that Ilsa and Nick have been trying for a few years to have a child with no success.
While a guest at their home, Ilsa (jokingly) invites him to a street party in celebration for Prince William and Kate Middleton’s wedding, but Strike declines.