South Korean shares marginally declined on Monday, as investors sought to book profits to end a four-day winning streak. Markets eagerly waited for the political scenario to unfold further, as acting President Han Duck-soo stepped in on Saturday following President Yook Suk-Seol's impeachment over his martial law attempt earlier this month.

The Korea Composite Stock Price Index, or Kospi, was down by 5.49 points, or 0.22%, to close at 2,488.97. The Kosdaq rose by 4.8 points, or 0.7%, to 698.53.

The Bank of Korea said Sunday that financial and foreign exchange market volatility eased after swift stabilization measures were taken and daily emergency task force assessments are underway following the martial law attempt by President Yoon Suk-Yeol earlier this month.

The passage of the impeachment motion in the National Assembly on Saturday, Dec. 14, improved political predictability, aiding market recovery, the central bank said. Stock prices rebounded quickly, while Treasury bond yields remained stable. The won/dollar exchange rate showed volatility but aligned with global dollar trends, according to the Bank of Korea.

The South Korean central bank pledged continued efforts to maintain stability, citing minimal long-term economic impact from past impeachment episodes.

In corporate news, the European Medicines Agency's Committee for Medicinal Products for Human Use (CHMP) issued positive opinions for Celltrion's (KRX:068270) biosimilars Eydenzelt (aflibercept), Stoboclo and Osenvelt (denosumab), and Avtozma (tocilizumab), Celltrion said Monday.

Eydenzelt, comparable to Eylea, targets retinal disorders such as wet AMD and diabetic macular edema. Stoboclo and Osenvelt are at par with Prolia and Xgeva for osteoporosis and cancer-related conditions. Avtozma is comparable to RoActemra for rheumatoid arthritis and other inflammatory diseases, the press release said.

The products require European Commission approval for marketing in the European Union.

Shares of Celltrion rose more than 1% at market close on Monday.

In other news, Asiana Airlines (KRX:020560) repaid 1.1 trillion won in loans via funds raised from Korean Air's (KRX:003490) capital raise, lowering its debt ratio to the 700% range from 1,847%.

The repayment included perpetual convertible bonds worth 680 billion won and operating loans worth 420 billion won.

Separately, Korea Investors Service upgraded Asiana's credit rating from BBB- to BBB0, the first such rating in seven years.

Shares of Asiana Airlines fell nearly 2% at market close.