Paul Singer earned his reputation as a take-no-prisoners brawler by challenging the interests of Argentina¡¯s government and South Korea¡¯s family businesses. With his latest investment, though, the American activist investor appears to be taking a more collaborative approach.

Singer¡¯s Elliott Management Corp. took a stake of almost $3 billion in SoftBank Group Corp., saying the Japanese company¡¯s shares are woefully undervalued compared with its assets. That¡¯s exactly the argument SoftBank founder Masayoshi Son has been making for years.

Son is finding himself pleading the case more frequently after a series of stumbles in recent months. WeWork¡¯s plan to go public imploded, forcing SoftBank to arrange a rescue financing of $9.5 billion in October. Uber Technologies Inc. continues to underperform. At least two managing partners of SoftBank¡¯s $100 billion Vision Fund have left since December, and the company is struggling to secure investors for its second technology fund.